There's one in Serpent Kingdoms called Deadly Poison that will double the secondary damage. There are also some other cool feats for poisonous creatures, like Spit Venom (makes it a ranged attack) and it's followup, Deadly Spittle, which turns it into a 15' cone breath weapon.
Savage Species also has the description for Deadly Poison. Additionally, SS has one called Virulent Poison, which increases the DC of the save.
Hope that helps.