Make sure you download 3G watchdog - keeps tabs on your download limits and warns you when you get close to it.
Another note - "In settings - accounts & sync" turn of background data when not using market,. Having this ticket eats into the battery life
Running live wallpaper shortens the battery life.
Other worthwhile apps to download-
Angry birds - addictive game
gauge battery widget - clearer way off watching battery power
Handcent SMS - better than stock messaging
Palmary weather - Excellent weather app
Picsaypro - Great photo editing app