One of the first OSR-type games I came across was Basic Fantasy RPG. It is not a retro-clone, per se, but it is it's own thing, based on the d20 SRD and injected with the simplicity of B/X D&D. It is a very cool and easy to learn game, is 100% open (except for the art, I think), and 100% free. It is available on Lulu in 3 different formats at cost. In addition, it is hosted on a well-maintained site, which also includes tons of free user-created adventures, rules supplements, and other stuff like reference sheets, character sheets and GM screens.
Out of all of the "open" games available, I'd say that BFRPG is the most open. All the documents are available in both PDF and OpenOffice format, so it is a great platform for creating a customized game of your own design, with most of the heavy lifting already done for you.
Mythmere's post brought my attention to the fact that I have never thanked Chris Gonnerman, and all the others associated with the project for all the hard work they've done in bringing a very high-quality free game to us. Matt started a thread over on Dragon's Foot to thank Chris, so if you've made use of his work, head over there and drop him a note.
I've used BFRPG to play games, and also as reference material for personal projects. Below is a photo from a BFRPG game I played with my sister and brother-in-law, in which they were assaulting a mommy dragon and her two brat children that were causing trouble in the region.
|Crazy zealot cleric just charges right in!|
Below is a pic of my homemade copy of BFRPG. I printed it at home on my laser printer, included a few supplements, made a custom cover on cardstock, and had it spiral-bound at Office Depot.
|Acetate cover flipped over to reduce glare|