Modern Times Bookstore is "San Francisco's oldest collectively owned & operated bookstore," and they're located in the Mission. As you might expect, they're a co-op. They have "Labor Days" where you can come in and commit to helping out around the store (sign up at the front counter).

You can join and become a member and receive discounts on purchases. Members don't vote, though, only employees do (is this true?), as it's a worker owned co-op.