Sparrow Lite is a handy tool for Gmail users that's exactly the same as Sparrow but is limited to just one Gmail account and features ads.
The discreet interface feels a little bit like the Tweetie Twitter client with a slick Cocoa finish and an easy to scroll through threaded message window. You view previews of mails in the main window and click on them to expand the full view. You can read them in a new window or open the right pane to enjoy Sparrow Lite's full potential.
An unread message in the counter sits in your Menu Bar to keep track of mails and Growl notifications let you know when a new message has arrived. Unfortunately, in this Lite version of Sparrow, you can only add one Gmail account and you will be pestered by ads from time to time.
In addition, some of the functionality of Gmail is missing. For example, you can't access filtering functions and you can't edit signatures. Finally, Sparrow Lite seems to be quite heavy on resources so watch out for it hogging your CPU.
If you're a Gmail user looking for a slightly different way to manage your account, Sparrow Lite is worth a go.