Many popular Social Networking websites have been converted into FREE iPhone Applications.  Here’s a short list

App Name App Functionality App Notes
LinkedIn LinkedIn website Groups are missing from the iPhone App
Facebook Facebook  
MySpace Mobile MySpace  
Y! Messenger Yahoo Messenger  Not compatible with desktop Conference
AIM (Free Edition) AOL Instant Messenger  
TweetDeck Twitter + Facebook Transmit to multiple Twitter and Facebook accounts simultaneously

For those that are iPhone users, are there other Social Networking Apps you use?