The second Art City Tour of 2010 is already here and is about double the size of the first one. Today, the 21st, starting at 6:00 p.m. you can visit over 20 galleries in the downtown area for free. Buses start out of MADC. My recommendation is to start at one of the gallery clusters away from MADC as the wait for a bus was about 45 minutes last time. You must prioritize because the event only lasts until 8:30 p.m., way too short to get to even half of the galleries.
I think the most interesting exhibitions will be found at Kalu, Fundacion teor/etica, and Casa de Cultura J Figueres.