If you just want a cheap way to scan RF (without being able to see individual packets) get an old Proxim RangeLAN2 card with their Site Survey tool and run it in 'Slow' mode - 1 second per MHz.
I use this! Locust™ is a wireless receiver designed specifically for sweeping and optimizing 2.4 GHz Local Area Networks. The instrument measures coverage of direct sequence CDMA networks which operate on the IEEE 802.11b standard allowing the user to determine the AP (Access Point), PER (Packet Error Rate), and RSSI signal levels aiding in locating the hub and access points of neighboring WLANs. Locust™ provides measurements in realtime and also logs data for further post processing analysis. Users may detect and differentiate from narrowband multipath interferences using a drive-test vehicle, GPS antenna and a laptop PC.http://www.bvsystems.com/Products/WLAN/Locust/locust.htm
