Airport Radar is a really useful widget that will help you find available wireless networks. It's created by the people at MacWireless.com, which is cool to me. It's almost like they are saying, ?We're so sure our products will increase your network strength that we're giving you a tool to test it.?
If you are into full apps, you can check out MacStumbler or KisMAC, but this widget has replaced both of those apps for me. I'll tell you why:

When you're on vacation (or just cruising your neighborhood) the widget is really fast to pull up and is always running.
It gives you all the info you need. Network name, channel, open or not, signal strength.
And my favorite feature is the ability to join a wireless network by just clicking on the name in the widget. That is really quick and quite nice.