Murrieta Homebrew Emporium is a good one. It's at the very back of a set of industrial buildings on Sky Canyon Drive, east of Winchester Rd and north of Murrieta Hot Springs Rd. I've told Jim (the owner) that he should give a free beer as a prize to new customers just for managing to find the place....
There's another one I can't remember the name of, a block south of the DMV office on Diaz Rd. It also has a good selection and friendly people, although it seems to me the prices are higher. But it's open on Mondays, and Jim isn't.
One of our homes is in Murrieta, and it's the one my wife likes to hang out at. But I work in Blythe, and do my brewing there when I'm off work but don't have enough time to make it home.