Mac OS X: Beta email client Sparrow is a minimal desktop app that easily plugs into Gmail — and any IMAP account in the future — and supports many of the handy things you'd hope for from a native email client.
That includes drag-and-drop attachments, Growl integration, a decent array of keyboard shortcuts (though we'd prefer if they were more Gmail-like, since Gmail is currently the only app they support) and so on. Its main selling point is that it's light and attractive. One serious problems for hardcore Gmail users: It currently doesn't support labels — though they're planning to add label support in a future release.
It's still a pretty young client (though if you're familiar with Tweetie, it probably looks pretty familiar), but it certainly has some promise. Currently, Sparrow is a free beta download for Mac OS X only.