While there are lots of online forums, it's best to try and get ones that are already big and active, since they are most likely to stay around for a long time. Also, they tend to have a lot of different categories including general or "lounge" sections in which you can post about pretty much anything. This is important, because after a while you do tend to get a bit bored writing comments about just one or two subjects.
Of course the SEO value of backlinks from these miscellaneous sections is probably even less than from from other parts of the forum. Still, a link is a link (although, of course that's not the only reason to post, of course!) and they really do add up over time.
As well as this you are helping the forum keep up steam by adding your thoughts. The more content that the forum has, the higher it ranks in the search engines. And the more likely it is that new people will join. So the cycle continues. (Also, you are generating some content that you can develop into articles or blog posts to use later on.)