My favorite Twitter app, Twitterrific, has released a major feature update chock full of refinements on iOS, bringing it up to par with the Mac app.
The biggest new feature is username suggestions when composing a tweet. Simply type the @ symbol then start typing either the username or the corresponding listed real name, and a little slide bar with show up showing you users that match. Tap the user and the name is filled. Other apps do this, but Iconfactory's implementation is pretty slick.
Also, Twitterrific's tap gestures have been simplified and overhauled. I'm especially loving the Report Spammer button when long-tapping an avatar. That button has been long overdue.