WifiInfoView is a compact tool which scans for wireless networks and displays a host of details on its findings.
This includes all the usual details, of course, like the network name, signal quality and security. But there's also the MAC address, frequency and channel, as well as the network PHY type (802.11a, 802.11g, 802.11n, or High-Rate DSSS). You may even see the router model and name, as well as the company who made it (although this depends on the router itself).
The report also includes the dates when the network was first and last detected, as well as the number of times it's been detected in total. This is great for picking out when something has changed (a new network has appeared, perhaps).
If you're a wifi expert then you can also select a network and see a hex dump of all the information received from that device.
But if you're a little less technical then you can alternatively choose a Summary mode, which drops most of the low-level details and concentrates mainly on the wireless basics.