It's not an app, but google sync is amazing. It keeps your contacts and calendars up to date with your gmail account and does so entirely over the air. Up to date contacts doesn't sound like that big a deal, except that it means you'll never have to back up your contacts again. If you lose your phone all your friends numbers are still available in gmail.

My favorite app is probably pixelpipe. It lets you upload your photos straight from your phone to flickr, facebook, picassa, or wherever else you want photos to end up.