Ask HN: Which Vim plugins / config line can you not live without?

I'm sure most of us know about the following plugins, so please don't include these in your response.

- SuperTab

- NerdTREE

- Surround

- MatchIT

- Snipmate

- NerdCommenter / TComment

- Ack

- Bufexplorer

- Ctrl-P / Command-T

- Syntastic

- Fugitive

A simple description of the plugin and how you use it would be great. Always willing to improve my toolset :)

PS: I've heard a LOT of good things about EasyMotion and Gundo plugins, but haven't really given them a fair shot. If anyone uses them, do let us know.