Meraki offers up a Java tool for viewing Wi-Fi networks around you: The Meraki WiFi Stumbler is a browser-based tool for using your computer's 802.11 adapter to scan the air around you and report the SSID, MAC address (BSSID), 802.11 standard (g, n, etc.), channel, signal strength, maker (if known from MAC address), and security standards used. This is the first browser-based tool that I've seen that replicates the functionality needed to make channel selection and siting choices.
Because of the Java requirement, the tool works only in browsers like Firefox, Internet Explorer, and Safari that support Java. For some reason, only Windows and Mac are currently supported; Linux is not, but that might relate to the state of monolithic driver support in the open-source kernel. You can use the stumbler without an Internet connection in Firefox 3.5 only.
I made a very short screencast showing how it works. Looks like I was wrong in what I said in the screencast: there were simply no 5 GHz networks in the vicinity for a long time (I spotted one later); my desktop Mac does have 802.11n capability.