iPhone only: Last month we showed your how to set up push Gmail on your iPhone in OS X or Windows, but if you didn't like running your PC all the time to get your notifications, GPush can help.
GPush is a newly approved iPhone application that, very simply, adds Gmail push notifications to your iPhone. (If you're unfamiliar with the concept of push notifications, GPush notifies you the instant you receive a new email rather than after your Mail application checks for new email [or pulls].) To use it, just fire up the app and give it your Gmail username and password (your login credentials are encrypted with SSL and sent securely to Google). After that, you'll get push notification (complete with the From and Subject fields) any time you get a new message in Gmail. GPush allows you to toggle sounds, alerts, and badge notifications, so you can determine what level of alert you want.
The catch: GPush will set you back $1 at the App Store (for now, at least—it goes up to $2 eventually). Still, a one-time price of $1 isn't bad for the potential time- and battery-saving advantages of push Gmail on your phone.
Update: Readers are having mixed results with the app, and the makers of the application are aware of the problem and working on a solution. It's still working fine for us, but your mileage may vary.
GPush [iTunes App Store]