When it comes to the OSR style games, I think Microlite74 frequently gets overlooked. It isn't a direct port of any of the original D&D rulesets - instead, it's roots are in 3e, condensed down to their most basic rules into the original Microlite rules, then flavored and mixed with some Old School sensibilities. Oh, and you won't be finding it at RPGNow. They are free (donations appreciated).
The latest edition and its various versions are available here: http://blog.retroroleplaying.com/
They are going thru final proofreading, which you, the good reader, can help with. I'm currently looking at Microlite74 Extended. Clocking in at just 381kb and 28 pages, it's amazing how they have squeezed a complete reworking of the original boxed set and the supplements into 28 pages. It's not OD&D, but it has much of the feel of OD&D.
I don't think it would work well as an introductory RPG, but in the hands of experienced gamers looking for a smaller ruleset to get their game on, this should work remarkably well.
The Atlas of Pern -
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