And if you're not willing to pay for a Twitter app—understandable!—Echofon isn't too shabby. It's super-fast and stupid-simple, so it'll do well by all but the most obnoxiously obsessed Twitterers.
What it lacks in bells and whistles, Echofon more than makes up for where it counts, at least for most: It's as quick as Twitter apps get, and caches Tweets, so you can read them later on without a connection. It doesn't support multiple accounts, but most reasonable people don't need that. (I'm looking directly at you, @mattbuchanan)