Rayko Photo Center

Rayko Photo Center mostly offers classes (I've taken a couple of excellent ones) but I thought I'd mention them here because they always have interesting (free!) little photo exhibits going on. There are lots of cool analog cameras too as well as an awesome photobooth. Their exhibits change fairly often and it's easy to check if there's something interesting going on on their website.
The front door is usually locked but just ring the bell and you'll be buzzed in.

Rayko Photo Center
423 Third Street,
San Francisco