I've used the Venus and uBlox GPS modules from sparkfun.com and I've been quite happy with them. They both have low-voltage serial outputs, so they're not plug-and-play with a desktop PC or laptop. You'd need to get one of the FTDI USB to serial convertors with the appropriate voltage to connect them over USB. These are both GPS receivers with WAAS, so they have an accuracy of a meter or two under good conditions.
You may also want to browse the ROS Sensors wiki page for a longer list of GPS and INS (inertial navigation) sensors that are supported by ROS.
I've also used systems from Applanix and Oxford Technical Solutions at work. They're quite good and provide incredible accuracy (<0.1m), particularly when used with a base station, but they're also very expensive.
Any one used GPS module?