Well, whats up doodz. Check out the radical Open Source project(s) of the inimitable GRL. They have masterminded many a sweet project in relation to public art, graffiti and free speech... g'wan and check it out at graffitiresearchlab.com!
My favorite project of theirs is this killer bike sound system called the Audio Tricycle... not really physical, but audio, graffiti... I've always wanted to do this with a sweet dub mixtape and a bmx bike!!!
I also think this Laser Tagging deal is pretty sick... its been around for a while, so chances are you've seen it... but nevertheless it's a neat DIY movement.