Earlier today, Twitter blocked the popular Blackberry client for Twitter; UberTwitter. This must have come as a huge blow for users who relied on the software for checking on their tweets, replies, messages and updating their status.
iPhone users can try out Echofon for Twitter (free) or TweetDeck for iPhone (free too) or the Official Twitter client for iPhone (free). If you are a Blackberry user, you can use the official Twitter client (free) or Seesmic for Twitter (free) or Tweet Genius (free).
Android users can search the market place for the official Twitter client or also use TweetDeck for Android which is available in the marketplace.