Is there a directory of open source hardware projects?

If you're interested in arduino, there is http://shieldlist.org. It's a wiki listing of all of the boards that can interface to the arduino board. All should be licensed as open hardware.

Several places publish open-source PCB design files:

• Olimex sponsors open-source PCB contest every month; all documentation (including the PCB layout) is posted on the Massmind
• Circuit Cellar magazine hosts design contests a couple times a year; often PCB layout and other documentation is posted on the Circuit Cellar "contests" page.
• The Open Circuits wiki has a list of open-source hardware projects.

Sometimes people post open-source hardware design files on one of the popular open-source software places -- Google code, Sourceforge, Launchpad, Github, etc.

A few places are trying to make it easy to buy open-source PCBs -- the actual, physical things you hold in your hands, not merely some computer file:

• BatchPCB will sell more copies of your PCBs to anyone else that wants one. And give you money. Why wouldn't you want that? BatchPCB discussion forum at Sparkfun. A database of PCBs other people have designed.
• Open Source Hardware Bank (early discussion) has a library of open-source PCBs
• Seeedstudio has discussed a library of open-source PCBs.
• Arduino Shield List links to the source of many Arduino shield designs. Some of those sources merely publish the PCB layout files; others, in addition, sell the physical hardware -- typically as a kit.

I found some open source hardware at

http://code.google.com/hosting/search?q=hardware

but there are many software projects in these results also.

The only way I found of getting hardware only results is searching for PCB files:

http://www.google.com/codesearch?as_q=file%3A%5C.(grb%7Cpcb%7Cpcbdoc%7Cbrd%7Cgto%7C057%7Cgbl%7Cgto%7Cgbo%7Cgts)%24