Wi-Fi networks are everywhere, but finding them often requires tedious use of the erratic AirPort menu in the menu bar, or a separately running application, like iStumbler [1], that shows more information than most people require. (iStumbler is great for learning more about and troubleshooting the local AirPort-space, however, and includes support for scanning for Bluetooth devices and revealing Bonjour services on the local network.)
Christoph Sinai's coconutWiFi [2] offers a simple menu bar indicator: a single dot. The dot is red when there are no networks in the vicinity, yellow when nearby networks are encrypted with WEP (Wired Equivalent Privacy) or WPA (Wi-Fi Protected Access) protection, and green if at least one unprotected network is in range. (Scanning isn't sufficient to find other protection methods, such as WPA Enterprise, which requires a login, or MAC (Media Access Control) lockouts, with which specific Wi-Fi adapters are allowed access by their unique hardware number.)
Click the dot, and a list of networks drops down, including the method of encryption for protected networks. An optional number next to the indicator displays the number of networks found. The software is a universal binary and a 219K download; donations are accepted.
[1]: http://www.istumbler.net/
[2]: http://coconut-flavour.com/coconutwifi/