Well, .us was used for one of the original and most successful domain hacks to date with del.icio.us which later moved to delicio.us.
Eventually the site moved to .com with delicious.com as any successful hack should to prevent traffic leaks.
There have been several other .us hacks used as word plays but the most important in my opinion are local, state and national government sites on the extension. My own state of Texas uses state.tx.us as the basis for most of the state branch sites. Texas workforce commission as an example -
http://www.twc.state.tx.us/
This use lends a lot of credibility and inherent trust to local and generic industry sites on .us in the U.S. I own a few generics, such as Insurance.us, and get regular emails asking about related government programs assuming the domain is owned by a government agency.