First impressions: Gmail
I've been playing with Gmail some today and, so far, I really like it. The "conversation" functionality is really nice. Basically, it treats e-mail threads like discussion threads with only one link visible in your Inbox (instead of 1 link per e-mail in the conversation). So if you sent someone a mail and the two of you have exchanged replies a few times, all of those messages are grouped together and accessible in a single click. Cool.