TouchDown provides push email for the T-Mobile G1 Android device
I've written about TouchDown for the Google Android a couple of times (here and here) and as I have said in those posts it gets better and better on a regular basis. I have been using beta versions and the latest available beta supports PUSH email, ability to send attachments, device-local drafts, and more. The folks at NitroDesk, Inc. were able to sign a licensing agreement with Microsoft to support direct push capability through Exchange ActiveSync. I think this is one of the first products for the G1 that provides this functionality, even though a couple others are working on it as well.