[pmoriarty]

Another great one was c2.[1] I've found a lot of really interesting programming-related discussions on there.

There's also Lambda the Ultimate (LtU)[2], which had about the deepest language development related discussions I've seen anywhere.

Yet another interesting one, quite different from the above two and from most anything else out there, was everything2.[3] It was kind of like Wikipedia, but not limited in subject matter to "encyclopedic" entiries, nor to "neutral point of view", so it has a lot of really opinionated, creative, and funny entries. Most people don't know about it, and it's been pretty inactive for many years now, but it does still exist and still has a ton of great content.

[1] - https://en.wikipedia.org/wiki/C2.com

[2] - http://lambda-the-ultimate.org/

[3] - https://www.everything2.com/