Labyrinth Lord. The first OSR game I found, assuming one doesn't count Castles & Crusades.
Certainly the first one I found freely available in PDF in it's special "no art" version.
Yes, there are non-free versions of the PDFs with art. Content is the same, so consider the art versions as a donation to the publisher.
Labyrinth Lord is a re-presentation or re-working of the B/X rules, and it tries to replicate the look and feel of such.
The core book for Labyrinth Lord is the LL Revised Edition. If you are looking to play something compatible with your old B/X modules, this is all you need.
LL: Advanced Edition Characters allows you to play the classes from AD&D using the LL rules. In some ways it is very much like S&W Complete, but LL:AEC requires the core Labyrinth Lord rules - it is not a stand alone product.
LL: Original Edition Characters brings the OD&D White Box emulation to the Labyrinth Lord Ruleset. Again, you need the core Labyrinth Lord rules to use this supplement.
There is a certifiable shit ton of support material for Labyrinth Lord. Where as Swords & Wizardry has spawned over a dozen derivatives of it's rules, doing a search at RPGNow for "Labyrinth Lord" leads to over 350 related products (S&W comes back with less than 140.)
Labyrinth Lord is probably the best supported of the OSR rulesets, with adventures, settings, classes, monsters and more just waiting to be used. And even though we always say that OSR products are 90-95% compatible across the various rulesets, Labyrinth Lord is 100% compatible to the largest amount of source material right out of the box.
Labyrinth Lord and Swords & Wizardry are probably the "Big Two" of the OSR clones and it's impossible to go wrong with either one.
Labyrinth Lord is published by Goblinoid Games
Labyrinth Lord: Original Edition Characters (Free PDF / Art PDF 3.95/ Print SC $8.95)
Labyrinth Lord: Revised Edition (Free PDF / Art PDF / Print SC $21.95 / Print HC $31.95)
Labyrinth Lord: Advanced Edition Characters (Free PDF / Art PDF $6.95 / Print SC $22.95 / Print HC $32.95)