2006-11-22: I have just learned of a new project, "USBtiny". It sounds like it does more than my design did, all with clearly GPL-licensed software.
I am no longer working on this project. I encourage anyone who is interested to continue it.
The board is compact and one-sided, with an area less than 2 square inches and a short parts list. However, it provides:
Programming speed is about 100 bytes/second.
Boards in the existing run are missing one trace. Its location is shown in the image on the right.
The patches to avrdude are only tested on Red Hat systems (RedHat 7.2, 9, and Fedora Core 1), but in theory can work on OS X and Windows thanks to the portability of libusb. My new AVR programmer software is tested on Linux and Windows XP.
Some links of interest:
The information below is a dumping ground of related files: The Eagle schematic and board files, plus png images of them. The firmware, suitable for assembly by avra. Patches to avrdude 4.0.0. Some photos of the assembled programmer. The pin assignment in the slave connector. Anything else I thought was relevant.