Windows Mobile/Blackberry: If you have a data plan with your phone, Tiny Twitter will help you keep tabs on your friends and send out tweets without overage charges.
Tiny Twitter is just customisable enough to make using a Twitter client on your mobile phone comfortable without being overwhelmed with unnecessary settings and options.
You can adjust how frequently Tiny Twitter will check for new tweets, toggle sound and popup alerts, and adjust the size of the client's tweet buffer.
When actually reading and sending messages you can retweet, direct reply, and perform any of the standard functions like following and unfollowing. In addition you can temporarily hide a user—not unfollowing them, just temporarily hiding incoming messages from them in the event of excessive chattiness.
Tiny Twitter is free and available for Windows Mobile and as a Java-based client for java-enabled phones like the Blackberry.