A good deal of Twitter's appeal comes from the fact that you can send and receive messages from anywhere you happen to be with your mobile phone. Twitter's own mobile service, http://m.twitter.com, is fine but basic. If you want to amp up your listening on the road, try a dedicated mobile client.

For the iPhone, Twitterific (http://iconfactory.com/software/twitterrific/) and Tweetie (http://www.atebits.com/software/tweetie), shown here, are popular.

For the BlackBerry, try TwitterBerry (http://orangatame.com/products/twitterberry) or TinyTwitter (http://tinytwitter.com).

Incidentally, you don't have to run the same mobile and desktop clients. It works totally fine to run, say, Twhirl on your desktop and Twitterific on your iPhone.