I set mine up to kill time between web design jobs.
Then one of the major cycling forums merged with another and overnight I got a few hundred new members who didn't like the merger. It's just built up from there really, and takes up quite a lot of my free time now. I make enough money to offset some of the server rental, but it's really a labour of love.
The great thing about CycleChat.net is that it brings people together offline too.
My first forum was created from a project I was working on on another forum. When the site/forum announced it would be closing, I moved my stuff over to its own forum to keep it going. Unfortunately, it kind of fizzled out, lol.
The new forum I'm building with XF exists due to my love of music and the desire to have an internet radio station as a hobby, but with a more user-friendly experience. (it also gave me an excuse to buy a license )
Created it because someone I know was making tonnes of money out of their very popular forum. Luckily i created it in the niche where i work in, so never felt like additional work. Surprisingly it got more and more popular with google sending about 85% traffic, and now i make a decent amout on money out of it after subtracting costs. Considering quitting my job for full time web administration now...