Definitely. My wife was a early member of ravelry, and watching their success made an impression on me. Before it existed, knitters were mostly members of some vbulletin-like board whose name escapes me. I suppose that a good way to identify these underserved markets would be to look for popular vbulletin/phpbb sites aimed at some particular hobby, and try to think of ways to make something more useful for those hobbyists.

Also, you don't have to limit yourself to the female half of the population. There are lots of industries with a significant male presence that use some crap software. I can't believe how bad some of the software is that I've used working for non-technical companies.