The Globalsat BU303 GPS receiver I ordered showed up just now, and I had to plug it in and take it for a spin around the neighborhood. As you can see in the picture, it really is that small, and plugs right into the USB port. It has a magnet in the base, so it ostensibly sticks to the roof of the car. (I haven't tried that yet.) It works beautifully.
I don't have any mapping software, but GPSUtility shows you a monitor of the output, position, UTC time, and calculates direction and speed (albeit on about a 10-second lag.) It uses a Prolific PL-2303 USB-serial bridge chip, drivers available for OS X, OS 8/9, and Linux. Windows drivers ship with it on CD.
Oddly, GPSUtility doesn't show altitude.