Diablofans has a nice list of all the gems.
Only been playing for an hour, but I got myself a pain enhancer just now, and testing it out on my wizard (at the cost of some precious %crit damage) its proc truly does tremendously enhance the pain of my enemies.
Seems like an obvious choice to me.
Gem of Efficacious Toxin also looks really good.
After getting that one I'm not sure which one to put into the third slot, though.
Which ones are you guys using, or planning on using?