To give a couple completely random examples of really cool sites with vibrant communities and tons of content that I've only just discovered in the past couple of years:

CRT Database has tons of technical information about CRT TVs, and is a resources that I've found essential for retro gaming.

18xx.games is a website for playing stock trading railroad board games online.

Standard Ebooks has nice electronic versions of a lot of public domain classics. This is where I go on January 1st to pick out some new public domain stuff from the 1920s to read for that year.

I think in the 90s and 00s it was easier to find this kind of stuff.