inSSIDer is a free tool to analyze wireless networks. While this tool may not be as extended as professional ones, it has a few nice features that make this thing a must-have. Add to it that it's entirely free, and you've got yourself a nice tool for troubleshooting wireless networks.
inSSIDer can be used to inspect wireless network and troubleshoot access points. It goes as far as viewing received signal (in dBm) over time, track channels in use, etc. There's also additional GPS support (NMEA v2.3) to locate access points, including support to export it to KML for viewing in Google Earth. The tool uses the native Wi-Fi API and uses a current wireless network card, without actually claiming it exclusively.
In short, inSSIDer is one of the better free wireless network analysis tools there currently are.