After getting an iPhone, I started looking around for the best Twitter client for the iPhone. There are many, many Twitter clients, attesting to the service's huge popularity. I tried Twitterfon and NatsuLion, both of which are good, but I like the feature-set and general snappiness of Tweetie from AteBits (8 bits in a b[i|y]te, get it?!). Tweetie's full set of features, from their website:
Handle multiple twitter accounts.
View your timeline, replies, direct messages and favorites.
Browse your friends and followers.
Post new tweets, retweet
Reply directly to tweets and send direct messages.
Follow and unfollow people.
Mark tweets as favorites.
Navigate reply chains.
Inline web browser.
Post links with automatic link shrinking via bit.ly.
Upload pictures to twitpic.com.
Update your twitter location.
View twitter trends and perform custom searches.
Save your favorite searches.
Implements the full twitter API.
Uses secure connection (https).
New: Go to User shortcut
New: Nearby search
New: Themes and adjustable font size
New: Bookmarklet support
Twittelator also looks pretty awesome, and I might try that after a while, too.