Find local access points with WiFinder.com: a truly useful site. If you're offering public access points, make sure and list them here, too.
Find whether you've been exposed: Netstumbler.com is listing networks that its users have picked up on their antennas.
Sprouting up everywhere: I turn on my Mac at home last night and find a new Wi-Fi network. My new neighbors are running unprotected 802.11b net. Time to introduce myself and talk to them about WEP and other security issues.