http://www.netstumbler.com/downloads/

 

The computer will do it as well. Just use the 'View Available Wireless Networks' built into XP.

 

You can also purchase inexpensive WiFi signal detectors such as this:

http://www.thinkgeek.com/computing/accessories/6f76/

This is just one of many. If you Google "WiFi Signal detector" you will obtain a list of them. These devices will allow you to detect without powering up your laptop. Some also provide signal strength indicators so you can walk around and find optimum signals. If you want to pay a little more there are also those that will query the server to see if it is unsecured so you can save additional time not powering up a laptop.

 

Another handy piece of software is WiFi Hopper.
http://wifihopper.com/