Politwoops, made available in the U.S. by the Sunlight Foundation, is an interesting (in some cases, also fascinating and useful) service that tracks and resurrects (undeletes, if you prefer) deleted tweets from U.S. politicians (congress, governors, etc.).
In other words, if a politician deletes a tweet it’s brought back online and made accessible (search or browse) to all via the Politwoops web site. Undeleted tweets are also indexed and searchable by Google.
In a recent blog post, Nicko Margolies from the Sunlight Foundation shares a look at some of the “best deletions” from Twitter during 2013.
Btw, Politwoops tracks the deleted tweets of politicians in many other countries including Australia, Canada, and the UK. Here are direct links to these services.