Here they are:
1. Blast Follow - indispensable for finding people in your niche (and beyond) and mass-following them with a single click. Good for research.
2. Tweepi - allows you to unfollow those who do not reciprocate. If you use this, you're using Twitter wrong.
3. twAitter - allows you to schedule your tweets in advance. Good app if you're using Twitter to broadcast blog postings. Not so good if used mostly for personal tweets.
4. Tweetdeck - allows you to organize your followers and keep an eye on the really interesting ones in separate groups. It also allows you to post to Facebook and Linkedin at the same time, and to schedule your posts as well. I use this one all the time.
5. Tweetchat - allows you to isolate the conversation based on the hashtag. If you don't know what a hashtag is, take a look at this past blog post.
For more information on each of these tools, see the original article.
Check out my Big Picture blog for discussion on common music, engineering and production tips and tricks.