I feel lucky to live in a city with excellent museums. San Francisco is not New York, London or Paris, but the San Francisco Museum of Modern Art isn't bad and their website isn't bad either. A lot of pieces were donated by the Fisher Family (founders of the Gap) and range from Miro to Kahlo to Picasso to Cornell to Calder to Rauschenberg.
You can see the collection here:
http://www.sfmoma.org/projects/artscope/
The have an sfmoma blog:
http://blog.sfmoma.org/
And one of my favorite paintings in the museum--a Rothko -- not quite the same online as in person, but...:
http://www.sfmoma.org/projects/artscope/index.html#r=878&artwork=*****&zoom=9