MOMA augments an exciting permanent collection with vibrant exhibitions, all housed in a delightful building of exo-art. The curation runs to the modern side of modern art (edgier than New York's MOMA), but it's a reputable museum and lags behind the cutting edge of urban art in SF.
With it's close proximity to SOMA and Downtown - both readily accessible by Muni and BART, the museum can fit nicely into a fuller day of activity. It's also a good cold or rainy day option. Coffee enthusiasts may appreciate the Blue Bottle cafe on the rooftop with the sculpture garden :)
Address: 151 3rd St, SF, CA
Website: http://www.sfmoma.org
Wikipedia: http://en.wikipedia.org/wiki/San_Francisco_Museum_of_Modern_Art