A popular use of WiFi in car computer setups is called 'War Driving'. This is a process where you run a program that looks for access points that allow you to access the internet. A typical use of that might be to have a program that periodically grabs a few K worth of data from a traffic site or occasionally uploads your GPS coordinates to a server for tracking.
War Driving, despite the name, is completely legal. Connecting to personal networks to access their internal data, however, is not.
The most popular program for doing this on Windows is called NetStumbler. It is available at
http://netstumbler.org/ and supports many popular chipsets. It can also use your GPS to assign a location of each access point found so you can maintain a database. Driving from work to home in Los Angeles, I typically see upwards of a hundred access points, half of them without WEP encryption enabled.
On Linux, popular programs include AirSnort and Kismet, the latter a tool that can also detect people running NetStumbler.