Netstumbler is the best known Windows tool for finding open wireless access points ("wardriving"). They also distribute a WinCE version for PDAs and such named MiniStumbler. The tool is currently free but Windows-only and no source code is provided. It uses a more active approach to finding WAPs than passive sniffers such as Kismet or KisMAC.
For downloads and more information,
visit the NetStumbler homepage.
Its a neat tool. It works excellent for mobile wardriving. It has audio alerts which are very handy while on the move. Unfortunately it only works on MS windows but I like to use dual boot OS's. Also it is only a discovery tool. So unless you find unsecured networks your work has just begun, but then even any run of the mill wireless adapter app can detect an unsecured network