HistoryOfWar.org - http://www.historyofwar.org/
Aims to make the site your first call for information on any aspect of military history.
Memory Alpha - http://memory-alpha.org/
A Star Trek focused encyclopedia that has user submitted content on many different aspects of the popular franchise.
PlanetMath - http://planetmath.org/
Mathematics encycloedia. Peer reviewed and user submitted with TeX input.
The Skeptic's Dictionary - http://www.skepdic.com/
Offers over 700 definitions and hundreds of articles on the paranormal, the occult, the supernatural, and the pseudoscientific written from a skeptic's point of view.
Trending articles - http://wikipedia.trending.eu/
Offers statistic on the most read articles currently on Wikipedia. Provides hourly updates and stats can be viewed over time.