Here’s a new mail application which I’m looking forward to using—Sparrow. The interface is clearly derived from the simplicity of the current Twitter for Mac versions, which I’m not crazy about, but is workable. The app itself is actually a GMail client, so you can use it to replace Apple Mail and the workarounds that come with that when using GMail as your primary mail account.
This is version 1.0, and as such there are issues, but the developer is responding quickly and pleasantly to those concerns. Available now in the Mac App Store for $9.99.