If you are a big online gamer and own a Xbox, 360 Live [$1.99 - iTunes Link] is the app for you. There are many apps within the App Store that perform many of the same functions as 360 Live but none of them are this well integrated.
- Sign in securely using your Xbox Live credentials. 360 Live automatically retrieves your friends.
- Complete Gamertags management. Add, remove, accept or reject pending requests.
- An organized Friends List. Grouped by status (Online, Offline, etc.)
- Gamer Profile and details including gamerscore, bio, location, country, gamercard, avatar, etc.
- Check your friends complete games history.
- Compare locked and unlocked achievements per game and gamertag.
- Send and receive messages. You can even send messages to multiple recipients!
- Gamercard coverflow style carrousel on landscape mode!
The only negative that I've seen testing this app out is that at times, it can take some time to load. Hopefully that is cleared up with an update or two and if it is, this will be hands down the best Xbox Live companion piece.