Gmail as lifesaver
For several years I've been a big user of Google Apps. All my email routes to Google's servers and with the great IMAP functionality I've considered myself fortunate not to need to run email servers much anymore.
A few weeks ago I moved from using the powerful Apple Mail and MailActOn combination to using the Google web interface. While I still sync up with IMAP every day or so and will continue to use Mail both for offline access (though this seems to be coming along in Gmail) and some advanced features like integration with iLife and other apps, Gmail has a few features that Apple Mail can't match.
One of the big features is an addition to the labs tab in Gmail which provides a seemingly simple "Send & Archive" button.
This deceptively simple button makes it very easy to reply to a conversation and archive the conversation. When somebody replies to the conversation it instantly pops up in the inbox again. Another great advantage is that because Gmail puts the conversation, and not just the new messages, in the inbox it's easy to see the whole conversation on my next reply.
Now if only Google Apps had better desktop integration and themes...