In D&D, some creatures have a bonus to saves against poison or immunity to poison. Personally, I think it would be handy if effects were labeled as "poison" if they were poison. For example, stinking cloud creates a bank of fog vapors that are nauseating ... which dwarves should be good at resisting, and demons (which are immune to poison) should be able to ignore.
So, with that in mind, "poison" should be a designater, just like darkness, light, fire, and so on. Magic items and monster abilities should be labeled as poison if appropriate, too.
I've put together a short list of spells that should have this designator (there may be some in other books, too). This list deliberately does not include spells that summon or create creatures that themselves have poison (such as a fiendish viper from summon monster I or the fiendish or celestial vipers from vipergout).
Ghoul touch (stench aspect only)
Prismatic sphere (green layer only)
Prismatic spray (green beam only)
Prismatic wall (green layer only)
Lords of Darkness
Handfang (see the errata)
Magic of Faerûn
Cloud of bewilderment
Prismatic eye (green ray only)
Masters of the Wild
Kiss of death