Yankring: if you were ever confused when deleting/yanking/putting (was that phrase in "0, "1, "2?) yanking is beyond helpful.

Syntastic: calls out syntax errors for lots of different languages.

Command-T: I've had some trouble getting this compiled and keeping it working when updating vim. But it's the how I open new buffers.

Solarized: is a nice :color. I use the dark.

Pathogen: seems to be the way to manage plugins. I started vim from textmate ~a month ago using Janus. This week I changed from Janus to pathogen and it's a lot nicer now. (all of tpope's stuff is great as zefhous us mentioned.)