Wifi-hog
Here’s a fun one, albeit kind of mean: http://www.coin-operated.com/projects/wifihog.html
It really messes with the relationship between private and public cyber spaces. It takes over public wireless networks, so that only the person using the wifi-hog can use it. While it seems to be intended to screw with people’s perceptions of ownership, I think it may be doing so in a way that is hostile to public interests.