I was using TweetDeck and would always looking up the username of the person I wanted to @ on the web interface and then copy paste that info into Tweet Deck. I was thinking how great it would if it would autocomplete when you started with the @ sign.
Settings > Use autocomplete for usernames
Helps when you look doesn’t it!
You can download TweetDeck from here:
and is also available as an iPhone / iTouch app.