I know this option won’t appeal to everyone, but it’s so handy that I thought I’d single it out and discuss it a little bit.
If you click the thumbnail to the left, look at the bottom of the preference pane for the option that’s labeled “Save files when focus is lost”. If you’re in TextMate and get an IM that you want to respond to, your document is saved as soon as you switch to your IM client.
It seems like such a simple feature, but I have a habit of hitting save all the time. When I’m writing, I tend to save as soon as I finish a thought. The ability to easily undo changes means that even if I save a mistake, I’m still not committed to whatever’s on disk. Apps and operating systems have gotten a lot more reliable, but my habit persists out of fear of losing a thought or a big chunk of information. With TextMate, my data’s always safe :)