NetStumbler is a tool for Windows that facilitates detection of Wireless LANs.
NetStumbler has had some coverage on this forum before: http://portableapps.com/search/node/netstumbler
However, NetStumbler has it's fair share of issues including: It does not run on Vista, and it is not Open Source.
Enter "Inssider", which was created as an open-source alternative to Network Stumbler.
It's interface is nicer than the similar open-source Kismet.
NetStumbler/Inssider is yet another one of those important network security applications that is a must for any network administrator.
It would also be nice to see a portable Aircrack-ng or Airsnort.
I'm sure you can all work out the use for these applications being portable.