AirMagnet
http://www.airmagnet.com
Sniffer Wireless
http://www.sniffer.com
The NetStumbler survey tool
http://www.netstumbler.com
A Mac version of NetStumbler
http://www.macstumbler.com
WaveStumbler, a Linux tool that locates APs and assesses their strength
http://www.cqure.net/tools.jsp?id=08