I've been looking at web tools that let you do things with Twitter. Top of the pile is Twitterment a search engine and trend analysis tool. In a similar vein Twitterverse is a tag cloud generated from twitter posts, with links to tweets (twits?) including the tags shown. (tip Datamining)
Finally, since the preceding post was about social bookmarks you might want to take a look at what seems to be the first Twitter social bookmarking service Twitthis
UPDATE: Also found Twittertroll
another twitter search engine.
Any other suggestions for Twitter tools welcome. I appreciate I haven't touched upon the DIY opportunities opened up by the various feed mashing services out there.