I probably use the plugin 'tComment' more than any other plugin. It toggles code comments, based on filetype. I often try to toggle comments by hitting 'gc' on machines that don't have that ability. Very simple, but has quickly become indispensable.

I keep a list of links to things related to Vim, and the list of plugins I use is at the very bottom, if you're interested: http://mileszs.github.com/