Two visualization tools for Twitter are introduced here: mentionmapp and twiangulate.
Mention Map is a Twitter visualization tool that displays the connections to a Twitter account. The tool is being upgraded but the original version is still available (click on the "classic link" at the bottom of the page.)
Twiangulate is another Twitter visualization tool that enables you to compare two or more Twitter accounts. The end result is a Venn diagram of commonalities as well as a table of the top followers.