After testing out a few tools from the 140+ Twitter Tools post on Mashable, I decided to post my favourites so far:

Twitter Karma - Displays your followers and the people you follow on a single page, in a better format than Twitter provides.
Twitter100 - Displays all the people you follow and their latest posts.
TwitBuzz - Shows links posted to Twitter in a Digg-like interface.
Tweetstats - An amazing site that analyzes your profile and provides you with a wealth of statistical information about your tweeting habits. It even includes a tweet cloud!