Great program to track wireless networks in your area.
There are two versions: Windows or Pocket PC. Both are available @ NetStumbler’s Website.
Update: Latest version is v0.4.0. Program is currenly stable. It runs on any windows computer and you will need a WiFi card to track WiFi signals. It will support tracking 802.11a, 802.11b, and 802.11g networks. Although this will not crack WEP for you it is still great for gathering detailed information about the WiFi singals in your local area. This program is 100% AdWare and SpyWare Free! Also keep in mind, when it is tracking signals you will not be able to use the internet.
Link: NetStumbler | FAQs