What the Hashtag?! - This site provides a relitively complete list of hashtags that are being used on Twitter. This is a great tool if you want to engage in conversation on Twitter.
BackTweets - This site can allow you to search for a full URL based on take a shotened URL formats that are commonly used on Twitter.
Friend or Follow - This tool analyzes your follow list to identify friends, followers, and fans.
TweetDeck - This is a multiplatform Twitter client that will allows you to intereact and converse on Twitter more efficiently.
Twitter Friends - This is a great tool to analyze your interactions on Twitter. This is great for the true geek who lives for data and numbers.
While this isn't a complete list of tools it is some of the best I have found. If you have additional suggestions feel free to comment on this page or message me on Twitter @mrohde