LessWrong
Since about 2007, there has been a growing community—both online and offline—based around the idea of “rationality”, i.e. the idea that one should (1) have beliefs that correspond to reality, and (2) execute actions in the real world so as to achieve one’s goals. LessWrong (LW) is a community blog about rationality that has been the main platform online for discussions about rationality. Learning about rationality is valuable in itself, but there is also an aspect of being in a “rationalist community” that appeals to many. In addition, besides discussions about just rationality, LessWrong is an excellent source of information about STEM fields in general.
Rationality
The most obvious way to use LessWrong is as a resource for rationality. The Sequences are considered to be the main written content on LW, but I think the following posts do a better job of wowing people:
“Diseased thinking: dissolving questions about disease” asks the question “What is disease?” and considers when it is appropriate to sympathize with someone or rather to prod them to fix their problems.
Books are declining in importance compared to material one can find on the internet; see “Yes, a blog.”
Community
LessWrong has often emphasized the community aspect of the site. For instance, “LessWrong as social catalyst” is a repository of anecdotes about how being on LW has helped with networking and other social realms.
One can also find posts like “Don’t Be Afraid of Asking Personally Important Questions of Less Wrong”, which details one user’s positive experiences about asking personal questions on LW—and getting feedback from the community.
“How do you approach the problem of social discovery?” describes one user’s experiences about making himself “stalkable” on the internet, which has led to people contacting him, which in turn has led to social discovery.
LW also has offline meetups all over the world. I myself haven’t been to one (the one in Seattle turned into an effective altruism meetup group), but I’ve been many effective altruism meetups (hosted by the same people who originally did the LW meetups), and my experience has been largely positive.
Cognito Mentoring was also originally announced on LessWrong (among other places). As described in my page on Cognito Mentoring, my experience with the mentoring service has been positive.
STEM in general
Even if the rationality aspect of LW does not appeal to one, LW still is an excellent resource for STEM topics. Indeed, it is possible to use LessWrong simply as a place to find other resources: some of the best content on LessWrong might just be pointers to other high quality resources.
“The Best Textbooks on Every Subject” was the post that first got me excited about LessWrong.
“References & Resources for LessWrong” is excellent.