People ACTUALLY interact! Unlike the atomized flash mob, this typically involves people playing some sort of 'game.'
Pillow Fight Flash Mobs
http://www.youtube.com/watch?v=IVms_sahfak
I like this video because they show how spontaneous the fight is, and how quickly people disperse. Basically people carry pillows, inconspicuously (how? It's a pillow...), and start a fight in the middle of the street with them then disappear in the crowd.
Capture the Flag
http://www.youtube.com/watch?v=PvHYgzg-oDE&feature=related
These tend to span an entire area, more of a net than a mob. These are dangerous (see video, kids running across traffic! An accident waiting to happen!). People attempt to capture a literal flag from a hidden location across the city and participants run around tagging people and trying to get the flag. Not a mob I'd want to do outside, maybe in a mall? Not city-wide as it is too dangerous.
"Light Saber" Flash Mob
http://www.youtube.com/watch?v=rUZgrL85OKs
This video is from a flash mob a year old today! Awesome.
People hide lightsabers under their clothes then gather in a central location and then have a huge 'battle' in the middle of the area and then clear out after the battle. A little dangerous if someone gets hit a little hard during the flash mob.
Follow the Leader
http://www.youtube.com/watch?v=9nMyC576c_Q
Basically a giant line. Not so entertaining, but it IS a flash mob, nonetheless.