I think a lot of the smaller sites, e.g. the language sites, writing, personal finance, music, motor maintenance and repair, would be hugely popular sites if they were better known, but frankly I just found this whole network two months ago and it's been around for a decently long time. It was pure chance that I stumbled across it at all.
Why doesn't Stack Exchange do some serious advertising? Commercials, billboards? Targeted web advertising through Google? Or does it already, and I'm just blind?