AirPcap is a technology the editor of this site has used. For a long time the ability to perform wireless sniffing and packet injection was limited on the windows platform. With AirPcap and its associated USB hardware you can now perform advanced frame level protocol analysis utilizing free tools.
Please read more about AirPcap (created by Cace Technologies) here:
Cace Technologies
And for some good tools try the following:
WireShark
Cain & Abel 