James Duncan Davidson linked to this Kensington WiFi Detector in his blog and it looks really handy. It’s range is kind of limited and it doesn’t talk about instantly it indicates a signal, so maybe warwalking is more likely than wardriving.
[UPDATE: Looking at the product’s Amazon page—interesting that they’ve teamed up with Circuit City to offer in-store pickup—I see that it has a button that checks for signal strength. That limits the utility of the device, but probably saves on power consumption. Also, you may be able to rapidly click it or even hold it down—it’s not obvious or explicit whether you can.]
[UPDATE (8/1/03): This review paints a less-than-appealing picture of the WiFi Finder’s utility. If it can’t detect the AP beacon signal, then it’s practically useless. Thanks, BoingBoing.]
[UPDATE (9/4/03): Here’s the product done right.]