Here’s my unofficial list of essential social networking apps for the iPhone & iPad:
Twitterific: http://itunes.apple.com/us/app/twitterrific/id284540316?mt=8
Twitterific Premium: http://itunes.apple.com/us/app/twitterrific-premium/id284542696?mt=8
Twitterific for Ipad: http://itunes.apple.com/us/app/twitterrific-for-ipad/id359914600?mt=8
Tweetdeck (iPad app): http://itunes.apple.com/us/app/tweetdeck-for-ipad/id364153769?mt=8
AIM (iPad app): http://itunes.apple.com/us/app/aim-for-ipad/id364193698?mt=8
Facebook: http://itunes.apple.com/us/app/facebook/id284882215?mt=8
LinkedIn: http://itunes.apple.com/us/app/linkedin/id288429040?mt=8
Digg: http://itunes.apple.com/us/app/digg/id362872995?mt=8
iReddit Free: http://itunes.apple.com/us/app/digg/id362872995?mt=8
Tumblr: http://itunes.apple.com/us/app/tumblr/id305343404?mt=8
StumbleUpon (iPad app): http://itunes.apple.com/us/app/stumbleupon/id364194133?mt=8
Feel free to post any others that have been left out in the comments section.