MentionMapp (from Vancouver) is a very cool way to see the spread and commonality of any username or hashtag mentioned on Twitter. You can then see how those relate to other topics and users helping you build a visual indicator of relationships.
Once the map builds, you can zoom in and out, click on a name or hashtag and the map recenters and expands again.
Click and hold to drag around the map for larger ones when you are zoomed in. You can share the MentionMapp with a simple retweet button since you must use oAuth to log in before using the service.
Luckily they do not send out tweets or make you follow them automatically like some services. Clicking the word MentionMapp takes you back to the default entry screen where they build a map about their product name.