The Mechanics' Institute is one of my favorite places in San Francisco.
The full service library is wonderful, as is having a comfortable place downtown to just sit, use the internet, and not be bombarded with life advice from baristas. The MI also puts on wonderful speakers series, writers workshops, and is home to the best chess room ever.
Membership is a mere $120 a year. I have an innate resistance to joining things, but this is one heck of a library.
This giving Tuesday, please consider the Mechanics' Institute.