Airpwn is a framework for 802.11 (wireless) packet injection. Airpwn listens to incoming wireless packets, and if the data matches a pattern specified in the config files, custom content is injected "spoofed" from the wireless access point. From the perspective of the wireless client, airpwn becomes the server.
Some images of airpwn in action at Defcon 12 are available - WARNING, some of these images are unsafe for those with tender sensibilities.
Latest release: 1.4 on 5/27/2009.