Current top sites used to enhance the use of Twitter are as follows.
is.gd – URL shortener. Link is for main site. Get the bookmarklet to be most effective.
TweepSearch – A search tool for the bio section of a Twitter user.
Twellow – The directory of Twitter
Tweet Stats – Data about a Twitter account.
Twitter Search – Used to find tweets that include terms or that are from certain geographic locations.
What tools are you using?