Firefox

Finally upgraded to Firefox 0.8 after much procrastination, and it's just getting better and better. (For the uninformed, Firefox used to be Firebird, and before that it was Phoenix. They kept the version ordering though). I didn't see any large improvements on Thunderbird 0.5, though, barring better stability and speed.

Try them out! It'll be the best thing you've ever done for yourself (and your PC).