A lot of people have talked about spell paths--spells of a similar theme as various levels, such as the summon monster spells, fireball and delayed blast fireball, invisibility and improved invisibility, and so on. It makes sense that a spellcaster that knows one spell in the path ought to be able to learn or create the next spell in the path, since the building blocks are mostly the same. This article is an attempt to address that.
    My breakdown of the spell paths is at the end of this page. I tried to keep the paths within a single school, despite similarities between some spells in different schools. For example, sending is a sor/wiz 5 evocation spell, and demand is a sor/wiz 8 enchantment spell that is the same as seding but with an additional compulsion effect. Sending and demand are not on the same spell path, despite their similarities. This limited scope has several benefits. (1) It's simplest. (2) It prevents specialist wizards from wanting to jump to spells in a path that may be in a prohibited school. (3) It lets spellcasters with the Spell Focus feat to reap the benefits of that feat for every spell in the path.
    At the root of this concept is that if a sorcerer or wizard knows a spell that is on a spell path, that sorcerer or wizard is on that path. A sorcerer or wizard can be on multiple paths at once without penalty, and doesn't need to explore every path he or she is on. This is mostly a terminology issue that makes it easier for me to explain stuff later.
    This document is a work in progress. I plan to add spells from other WotC books to the paths, and I have a few spells in mind that will help fill out some holes in the paths. Feel free to discuss this in my message boards.

    Paths are presented in this format:

    The Path of Light:(0) dancing lights, light; (2nd) continual flame, daylight; (8th) sunburst

    The first part is the name of the path. Listed after that are the spells in the path, in order of spell level (lowest to highest). A spellcaster does not need to know all spells on a path (for example, a wizard might know dancing lights and continual flame, but not either of the other 0- or 2nd-level spells on the path).
    (The 2nd-level list for this path has two spells listed, separated by commas; both of these spells are on the path, and you can skip one or both, and having either counts toward being on the path.)

Paths by name of first spell in the path. Paths are listed alphabetically by the first spell in the path.

Paths by path name. The Path of Charm is listed before the Path of Disguise.

Paths by lowest spell level. Paths that begin with 0-level spells are listed before paths that begin with 1st-level spells.

Choose one of these two options for path magic for the sorcerer class. All options described below apply equally well for bards.

Low Power: Allow a sorcerer to "upgrade" to a higher-level spell on a spell path when a spell known of an appropriate higher level is available; the sorcerer can then replace the original spell known with any other spell of that level. He doesn't have to upgrade through every spell in the path (in other words, he can skip the upgrades to certain spells if he wants to). A sorcerer can make one upgrade per class level (even if he has multiple options for upgrading at a particular level).
    The result: sorcerers don't get stuck with the choices they made at lower levels, which might not be appropriate at higher levels (a Sor12 doesn't have many opportunities to use sleep, for example), but retain the same level of power overall.
    Example 1: Vond is a Sor5 who knows the invisibility spell. Upon reaching Sor6 he gains a new 3rd level spell known. Invisibility is a 2nd-level spell on the Path of Invisibility and invisibility sphere is a 3rd-level spell on the Path of Invisibility. Vond decides to upgrade his invisibility to invisibility sphere, which uses his new 3rd-level spell known slot. He now has an open spell known slot at 2nd level (formerly occupied by invisibility), which he may use to learn any other 2nd-level spell on the sor/wiz spell list. He chooses cat's grace.
    Example 2: Barant is a Sor7 who knows the invisibility spell. Upon reaching Sor8 he gains a new 4th level spell known. Invisibility is a 2nd-level spell on the Path of Invisibility and improved invisibility is a 4th-level spell on the Path of Invisibility. Vond decides to upgrade his invisibility to improved invisibility, which uses his new 4th-level spell known slot. He now has an open spell known slot at 2nd level (formerly occupied by invisibility), which he may use to learn any other 2nd-level spell on the sor/wiz spell list. He chooses Melf's acid arrow.

High Power: Every time a sorcerer gains a new spell known, check the paths he is on. If any of the paths he's on has a spell at the same level as the new spell known, he automatically gets the spell on that path as a spell known in addition to the one granted by Table 3-17: Sorcerer Spells Known in the Player's Handbook. For example, a sorcerer that gains a new 3rd-level spell known can search the paths he's on to see if any of those paths has a 3rd-level spell; if so, he gets that spell as a spell known in addition to any spells known granted by Table 3-17.
    Let's call these extra spells from along a path "bonus path spells known," or BPSK. The sorcerer doesn't have to pick every spell in the path (in other words, he can skip a certain spell in a path if he wants to, particularly if he has a choice of two or more options at a particular class level).
    You can only get one BPSK per class level, even if there are multiple paths with appropriate spells. This also applies if you gain new spells knows at different spell levels (like sor6, where you gain a 0-level and a 3rd-level spell known): you still only get one BPSK per class level.
    The result: sorcerers are able to proceed along a path without devoting multiple spells known slots to mastering a path, allowing them to branch out a bit and be more versatile. This makes the sorcerer more powerful than the norm (since he's getting up to 19 extra spells known over the course of his career).
    Example 1: Vond is a Sor5 who knows the invisibility and levitate spells. Upon reaching Sor6 he gains a new 3rd level spell known. Invisibility is a 2nd-level spell on the Path of Invisibility and invisibility sphere is a 3rd-level spell on that path; levitate is a 2nd level spell on the Path of Flight and fly is a 3rd-level spell on that path. Vond chooses invisibility sphere as his BPSK for sorcerer level 6, and picks some other sor/wiz 3 spell as his normal 3rd-level spell known. He could have chosen fly as his BPSK for sorcerer level 6 and picked another sor/wiz 3 spell with his normal 3rd-level spell known, but he couldn't have chosen both invisibility sphereand fly as BPSK because you only get one BPSK per level in sorcerer. He could use his normal 3rd-level spell known (from Table 3-17) to learn the path spell he didn't choose, but it's probably smarter to wait until sor7, where he gets another 3rd-level BPSK. (This example ignores that Vond gains a new 0-level spell known at sor6, since it's much smarter for him to use his BPSK for a 3rd-level spell than a cantrip).
    Example 2: Barant is a Sor7 who knows the invisibility and summon monster II spells. Upon reaching Sor8 he gains a new 4th level spell known. Invisibility is a 2nd-level spell on the Path of Invisibility and improved invisibility is a 4th-level spell on that path; summon monster II is a 2nd level spell on the Path of Summonings and summon monster IV is a 4th-level spell on that path (it doesn't matter that there are 3rd-level spells on both of those paths because it's OK to skip spells in a path). Vond chooses improved invisibility as his BPSK for sorcerer level 8, and picks some other sor/wiz 4 spell as his normal 4th-level spell known. He could have chosen summon monster IV as his BPSK for sorcerer level 8 and picked another sor/wiz 4 spell with his normal 3rd-level spell known, but he couldn't have chosen both improved invisibility and summon monster IV as BPSK because you only get one BPSK per level in sorcerer.

Choose one of these two options for path magic for the wizard class.

Low Power: First, any spells of a path the wizard is on are treated as spells of a specialized school for that wizard: +2 to Spellcraft checks to learn the path spell, +2 to Spellcraft checks to research a new spell on that path. (Note: The second bonus isn't technically in the core rules--specialists don't get a bonus to research spells of their specialized school--but it probably ought to be.) If the wizard is a specialist and the spell in question is of her specialist school, these bonuses stack (+4 and +4 total).
    Second, the cost of researching new spells on that path is reduced. Take the spell level of the highest-level spell known on the appropriate path and multiply it by 500 gp; this is subtracted from the research cost of the new path spell. A wizard who wants to create a mass improved invisibility spell (which should be 9th level, and obviously is appropriate for the Path of Invisibility) would normally have to pay 9,000 gp in research fees (1,000 gp per week, one week per spell level). If she knew the mass invisibility spell (7th level on the Path of Invisibility), with this option she could reduce that cost by 3,500 gp (7th-level spell x 500 gp) for a net cost of only 5,500 gp. If she only knew invisibility, she would only be able to reduce the cost by 1,000 gp (2nd-level spell x 500 gp).
    The result: the wizard doesn't necessarily have any more spells than normal in her spellbook, but it's easier for her to learn path spells and if she does a little legwork (research) she saves some money by pursuing paths.

High Power: When a wizard gains a level, she may add one spell from a path she's on to her spellbook for free. This is in addition to the two spells she may add to her spellbook for free upon gaining a new class level. If the wizard is a specialist wizard and the path spell is from her specialist school, it counts as the one spell from her specialty school that she must choose as one of her two free spells at each class level (so she gets to choose two free spells in addition to her free path/specialized spell).
    The result: the wizard is rewarded for pursuing a path and can use her freebies on other kinds of research. A wizard may end up with up to 19 extra spells this way if she plans carefully.