iStumbler is a free, Snow-Leopard-only tool that helps you sniff out nearby Wi-Fi networks, even if you're just stumbling around with an open laptop.
iStumbler has a clean and helpful interface: on launch, it automatically displays all networks in your vicinity in a chart that shows each network's name, mode, protocol (e.g., 802.11n vs. 802.11g), security (anything from Open to WEP to BSS), noise, channel, frequency, MAC address, vendor (e.g., Apple or Netgear), sample, geographic location, and last update time. You can move these columns around and sort the networks any which way you want (such as by signal strength), and a live graph charts signal over time for selected networks. Each time you join a network (there's a join button on the upper left), iStumbler creates a standalone, semitransparent graph window to track its strength. A pane on the left also lets you see only Bluetooth and Bonjour networks, as well as look through log and location data, with the option to quickly pull up map info in a variety of formats, including Google Maps and Geocaching. This location data also lets you roughly track your movement, taking advantage of Snow Leopard's GPS-like features (much like a similar feature in the original iPhone).
iStumbler is a useful tool for rudimentary network maintenance and troubleshooting, but it's also a great choice just for finding open networks to join, for anyone who wants more information than you'll find in the default AirPort drop-down menu.