I lived in Sacramento up until a few months ago, but there never seemed (to me) to be much there. That said, check out the Fox and Goose, downtown. They have a decent (and usually crowded) open mic night every Monday, hosted by Dave Baldwin, who is a top-notch guitar player, and you frequently see some pretty good musicians there (although, obviously they're not all folkies). On every third (I think) Thursday night, there's also an open Irish jam, which is pretty decent, though not at the level of some I've seen in Cincinnati. I've heard that there are some open blues jams, too.

Try checking Craigslist if you don't hear much from people here. There's some decent stuff out there if you look for it, but it can be hard to find.