[*]Twitterific - I've never used Tweetie (suggested above) but I like Twitterific a lot.
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I don't even know if the OP uses Twitter but, oh well, because I'm going to explain that:) Tweetie 2 costs $3 and Twitterific has both a free version and a $5 version.
The interesting thing, however, is that Twitter itself just bought Tweetie and the guy who writes it. What WAS going to be Tweetie 3 is instead going to come out as 'Twitter for iPhone' and be the official Twitter app. (Unlike Facebook, Twitter never had their own app before. They just let 3rd party companies make them.)
So my advice would be to buy Tweetie 2 for $3 and learn it. Because in a few months, the official Twitter app will come out and will (probably) be free. And at that point I'd guess that the official Twitter app will be the best choice. (
Just an assumption!) So why not get used to using the program that will soon evolve into the official program?
Ok, that's way more thought than needed to go into this. Just thought I'd share.