Wifi direction/location finding

Is it possible, by measuring the signal strength at different locations, or other means, to ascertain the approximate location of a wifi signal?

What software would you recommend to do this? (For me, OSX, but feel free to list others)

For OS X there's Kismac.

Another one for Windows that looks cool (I've not had a chance to try this one yet) is Ekahau HeatMapper.

See Vistumbler Detects Wi-Fi Hotspots, Maps Them :

Windows Vista only: Free, open-source application Vistumbler scans wireless networks within range of your Wi-Fi antenna, tracks signal strength over time, and handles pretty much everything else that other apps of this kind do. What sets Vistumbler apart from the rest is its support for GPS and live Google Earth tracking, which plots hotspots on a map.

This app is currently free on the App Store. It seems to be more suited to mapping signals within a building rather than war-driving, but in case that's what you're looking for, check it out.