www.folkjam.org is a growing list of user contributed jam sessions. Anybody can post the details of jam sessions, though they do review every post to make sure it is appropriate for the site and not spam. Just enter your zip code and see what you get!
Note: I've had various problems with different browsers when I use this site. Some don't load it at all, and on others I hade to refresh the browser to get it to load. So if you go to the site and see nothing, don't assume that the site is down.