OpenStreetMap
I bought a cheap GPS on EBay. I got a Palm Vx with a Magellan GPS companion attachment for ten dollars. By PDA standards the Palm Vx is completely obsolete, but it works fine as a GPS data logger.
The accuracy is pretty good. I have no way of measuring it explicitly, but it usually matches Yahoo aerial photos and Google Earth within a few feet. That is for open skies, either from my jacket pocket while bicycle riding or on the dashboard of the car. Looking at my tracks in Google Earth, it does correctly match the parking space in which I parked the car at the shopping center. Scary.
It didn't come with any software, so I'm using cotoGPS as the track logging software and converting it to GPX with gpsbabel. It appears that all the Palm-related GPS or map commercial software companies are no longer selling the products now.
Discussion
Comment from fröstel on 4 March 2008 at 10:59
It's possible to get an idea about the accuracy of your GPS. Chose some outdoor location where there is some kind of a fixed line on the ground. Go exactly along that line and save your log. Do the same a few days later a different time of the day. Then when you've got a couple of tracklogs of that line you can compare them...