Without a doubt the most used reside in my "Social" folder:

App: Linkedin
Rating: ✩✩✩✩ (out of 5)
What is it?: This is an iPhone truncated representation of your Linkedin account.
Cost: Free
Comments: I great reference tool and messaging tool. Be careful with the connections option cause it can "Download All" and if you have a lot of contacts it can kill your address book.

App: AIM Free
Rating: ✩✩✩ (out of 5)
What is it?: AOL's instant messaging service on your iPhone.
Cost: Free
Comments: Good app, but don't forget to log-out when your done otherwise your drunk buddies will AIM you stupid shit in the middle of the night.

App: Mashable
Rating: ✩✩✩✩ (out of 5)
What is it?: This feeds aggregator allows for a great mobile representation of the full website.
Cost: Free
Comments: Must have if you have the insatiable needs to be in the know.

App: Facebook
Rating: ✩✩✩✩✩ (out of 5)
What is it?: The Facebook website to your iPhone.
Cost: Free
Comments: If you're addicted to Facebook then it's already on your phone.

App: TweetDeck
Rating: ✩✩✩✩✩ (out of 5)
What is it?: This iPhone version of the twitter/social application is a great tool to organize, sort and search tweets and social posts.
Cost: Free
Comments: A must have if you use or enjoy twitter and find it a useful tool in your day-to-day efforts.

App: iCoolhunt
Rating: ✩✩✩ (out of 5)
What is it?: A social 'trend hunting' social application. Allow you to photo perceived trends and submit to the overall community for review.
Cost: Free
Comments: Fun app, though admittedly I need to spend more time with it. What can I say, not many trends in "happy-town."

App: Bump
Rating: ✩✩✩ (out of 5)
What is it?: One of the default must have apps for the iphone allowing you to "Bump" iphones together and securely pass user contact information.
Cost: Free
Comments: Pretty good app, doesn't always work the first time though. Additional note that you look like a fucking tool when you use it.

App: Loopt
Rating: ✩✩✩ (out of 5)
What is it?: This iPhone application will tell you where your other friends are that are within your profile on a visual map. Big Brother, you have arrived.
Cost: Free
Comments: I've watched this app from a far. It's starting to mature. We'll see what moves it makes in the next six months as I assume Facebook will move into the proximatiy based, "check-in" spectrum.

App: Foursquare
Rating: ✩✩✩✩ (out of 5)
What is it?: Lastly, the premiere location based iPhone application. Allowing you to create an account and find offers, review and fraternize locations with a game-like rewards structure. What's unique about location (or proximity based) applications is it knows your locations based up on your GPS within your phone and serve content applicable to your geographic location.
Cost: Free
Comments: Crack on a stick.