Some lovely chaps gather 'round to play this glorious game in Golden Gate Park fairly often. Check out their message board for more details.

Their description:

A most noble tradition invented in Ireland 117 years ago! Mallets and balls provided. Scorekeeping forbidden. Everyone is welcome. Bring your refreshments and your bike — anything that can take a couple bumps, wider tires are easier than skinny, fixed-gear is good, drop handlebars are not.

Bike polo is also played on Monday nights in Dolores Park. You can find them on the tennis courts.