Textmate is a great editor with lots of plugins and bundles, hoping to find a few more that will make Textmate even more useful.
I personally can't live without Ack in Project (http://github.com/protocool/ack-tmbundle), the built in search for textmate is awful, ack in project does searches 100x faster.
What's your favorite Textmate plugin or bundle and what makes it so great?
What kind of work are you doing? I have a number of bundles I use, but they're all language and framework related, not general bundles like you pointed out (useful almost everywhere).
Given that i code mostly in Python and R, those are the two (sets of) bundles i use most often. There are a few others i use that i'll mention here though because they might be of interest to a larger group. I'll also try to recommend lesser known, or at least lesser-used bundles, i.e., those outside the official repository.
The first is Pygments. If you use TextMate and you use Pygments, this is an excellent time-saver for 'Pygmentizing' code snippets. (If you don't know Pygments, it is a syntax highlighter written in Python, useful for e.g., highlighting code snippets for insertion into blog posts, and the like.) This Bundle is available on TM's Github repository.
Another favorite Bundle is Man Pages, also a small utility-like Bundle, rather than a language bundle. Highlight a shell command or just put your cursor on it, call up the Man Bundle > "View Man Page" and you'll get the full text of the man page for that command in an HTML window separate from your script, with proper headers and other formatting, so that you can actually read it--no rtf symbols, no scrolling, etc.
I love GetBundles, but it might be a bit meta for this question.
It lets you grab other bundles from GitHub and SVN, and installs them for you. Basically, it's by far the best way of getting new TM functionality.
I also really like the ProjectPlus plugin, although it isn't actually a bundle :-)
I just got hold of a TM plugin called Minimap, it gives you the minimap feature from Sublime Text (http://www.sublimetext.com/)
Here's the TM plugin, http://julianeberius.github.com/Textmate-Minimap/
ProjectPlus is also pretty nice.
Ack in Project has been replaced by AckMate (by the same author).