LessWrong (LW) is a web site and online community dedicated to the study of rationality. It includes both a community blog (any member with sufficiently high "karma" can post) and a Mediawiki-based wiki. The two areas do not share accounts.
Co-founded by Eliezer Yudkowsky. Part of the Singularity Institute. Inspired by the Overcoming Bias (OB) blog, to which Yudkowsky was a frequent contributor prior to the creation of LW; his OB posts now redirect to LW, where they have been reposted.