On the corner of 18th and Lexington St in Mission District there is an absolutely awesome mural called "Generator". It was made in 2003 by two artists:
Andrew Schoultz and Aaron Noble.
I still did not manage to figure out what authors wanted to express through it... I guess next time instead of focusing on details, I should look at the big picture.
The mural is painted in color, but I like it more in black and white: