Sparrow is a sexy, minimalist (but still very beta) new email client for Mac
Sparrow may outclass Apple Mail in looks, but it's got some notable weaknesses. First, it only works with Gmail (and Google Apps for your domain). Second, it doesn't support some of Gmail's most important features, including filters. I also noticed some syncing problems, but they were easily fixed by deleting and re-adding my accounts.
Keep in mind that Sparrow is still in beta, and it's got the kinks to prove it. Support for other IMAP accounts is planned in the future, though, and the few minor issues it's got should be resolved before it goes 1.0.