I’m an avid Gmail user, and while I love the Sparrow app for Mac I know that some people may be looking for a more traditional Google-style interface. I was browsing around the Mac App Store when I stumbled across a free app called MailTab (Mac App Store link). It’s a simple app that supports just one email account, and sits in the menu bar.
The interface for MailTab is merely the mobile version of Gmail, but it is capable of doing all of the most common things you’d need to do: read emails and threads, respond and compose messages, and search for emails.
MailTab doesn’t add much to the mobile interface aside from things like background mail checking so that you can get visual/audio alerts of new mail messages at the interval you specify. Plus you can use the Command+G keyboard shortcut to activate the app at any time.
There is a MailTab Pro version available that also lets you use Gmail chat, but the free version should be more than sufficient if all you want it for is checking email.
MailTab on the Mac App Store (Mac only; Freeware)