Xirrus Wi-Fi Monitor
If you use Vista and WiFi, Xirrus Wi-Fi Monitor will be one of the most useful Vista gadgets you can download. It automatically shows your signal strength, the SSID of the network to which you're connected, your current IP address, and shows you visually, in radar monitor-style fashion, the network to which you're connected and any other nearby networks.
That's just the basics. You can get a lot more information than that as well. Click the Show Details link, and you'll see in-depth information about the network to which you're connected, and any nearby networks. You'll see each network SSID, whether it's encrypted, the kind of encryption used, the channel it uses, MAC address, and more.
Note: This link takes you to the vendor's site, where you can download the latest version of the software.