If you have a neighbor that likes to borrow your open wireless access point, you can have some fun with them. Redirect all of their browsing, invert the images, or return blurred images.
The first approach I took was to establish a transparent squid proxy on my FreeBSD firewall that I have providing DHCP and NAT for my internal network. I added a third NIC to provide these services for my untrusted wireless network. This will also help you to segment and protect your wired LAN.