I use [Twitterific on my computer. I also like [url=http://www.tweetdeck.com/beta/]TweetDeck](http://iconfactory.com/software/twitterrific).
On my iPhone, I've tried dozens of apps, but really like TwitterFon best. It's blazingly fast, easy to use and simple.
I also use HelloTXT (and the mobile version) to post status updates so they'll post to Twitter, Facebook, etc.