While client software drivers are a good starting point, there are a number of free (or nearly free) utilities that will assist you with site surveys and link analysis. Here are some of the most popular 802.11b network tools today.
By far the most popular wireless discovery tool on the planet, NetStumbler provides a wealth of information about available networks. It will show you ESSIDs, AP MAC addresses, channels, WEP status, and more. It can show the relative signal and noise of a particular AP, making it handy for setting up point-to-point shots. It is only available for Windows.
This is a beta version of a NetStumbler-like application for Mac OS X. Like its Windows counterpart, it shows all available networks and the channels they are using. While it does a fair job of network discovery, currently it isn't particularly useful for setting up point-to-point shots, as it doesn't display a high-resolution signal strength meter.
Kismac is another Mac OS X application that uses the RF Monitor mode of the AirPort and Prism 2 cards to observe networks. It also allows the user to send raw frames to any AP or client and has a handy real-time graph of available networks and their relative usage.
Wavemon is an excellent ncurses-based network monitor for Linux. It has a number of useful features, including real-time signal and noise levels, an AP scanner, and a full-screen moving histogram. This helps tremendously when trying to find the other end of a long-distance link.
If you're looking for an exceptional raw RF monitor, look no further. Kismet can find APs (including APs running a closed network), send and receive raw frames, and even detect the presence of people running NetStumbler. Its sophisticated data logger allows export of raw 802.11 frames to tools such as tcpdump and Ethereal for later analysis. It currently runs only on Linux but versions for Linux-ARM, BSD, Windows, and OS X are under development.
These tools, combined with a good network analysis package (such as Ethereal), can help you find out a tremendous amount of information about how wireless networks in your area are being used. Keep them with you on site surveys, and you'll be able to do a much better job of planning your network.