If I were to check my browser history, RationalWiki would probably be sitting somewhere near the top in terms of number of visits over the past few months.
RationalWiki was originally formed as a counter to Conservapedia, a heavily right-wing version of Wikipedia. Since then, its mission has expanded to supporting rational thought and the scientific method against pseudoscience and in some cases religion. It is superficially similar to Wikipedia, as is any website based on the MediaWiki engine; however, RationalWiki mostly focuses on topics of interest to scientists and skeptics. It also abandons any attempt to write from a neutral point of view and often will be very sarcastic towards ideas which the editors regard as foolish. One of its other unique features is the set of "WIGO" (What Is Going On?) pages; these list recent happenings in Conservapedia, the mainstream news media, and various blogs the editors monitor.
I regard it as an extraordinarily useful and generally trusted resource, particularly with regards to researching sources and whether I can trust them to at least try to be unbiased. The WIGO pages in particular are a good place to find new and interesting blog posts and news stories. Politically, I would say there is a general left-wing slant, although RationalWiki's mission, like my own standards, is accepting of any point of view that can defend itself with a proper argument.