Swords & Wizardry is about as stripped-down as you can go, which makes it incredibly easy to add things back in. AC is dual statted (up and down), so if you prefer "3e-style" AC (I do), it's already built in. You can add feats and skills from any edition without any difficulty that I can think of. Land of NOD blog has mountains of custom tweaks and additions to S&W, both in the NOD ezine or spread throughout the blog.
To be honest, I've liked S&W for awhile, but seeing it "in use" was what really sold me.
1. Swords & Wizardry - It also has the single save, as an alternative to the five categories, if that's something you might prefer. It's my own favorite of any D&D save system. Free PDF.
http://www.lulu.com/shop/matthew-finch/swords-wizardry-core-rules-pdf/ebook/product-15718826.html
2. Swords & Wizardry Whitebox might suit your purposes, as well. You could take either one, put in an evening or two of work and have a nice, old school variant, with as much d20 influences as you would like. Free PDF.
http://www.lulu.com/shop/matthew-finch/swords-wizardry-whitebox-rulebook-pdf/ebook/product-14956259.html
3. Ah. The Microlite family of games might do the trick! Microlite 20, et al:
The Microlite20 RPG Collection | RetroRoleplaying
Microlite 20 is an ultra lite, stripped down d20 variant. Again, even if its too lite, it might make a useful starting point, for you to take and go as far as you would like. Free PDF.
4. James Bobb's World of Onn. I have an earlier version of this, which uses ascending AC, the d20 saves, a small list of feats, etc. Haven't read this version, but I suspect it's along similar lines. Might want to check with the author.
http://www.lulu.com/shop/james-e-bobb/1001-world-of-onn-core-rules/paperback/product-6500536.html
5. Have you checked out Basic Fantasy?
http://www.basicfantasy.org/downloads.html
It's along B/X lines. Ascending AC. Plenty of support. PDF's are free.
6. The Myth & Magic Player's and DM's Starter Guides are available for free PDF download here:
http://www.newhavengames.com/
Kind of a 2e & 3e mashup, I think. Or a 2e, with some d20 mechanics. Or, something. Haven't done more than glance at this one.
As for non D&D derived stuff:
7. Barbarians of Lemuria - Here's a link to the free version. An expanded for-pay version is also available:
http://www.1km1kt.net/rpg/barbarians-of-lemuria
I've got the expanded print version of this. Itching to run it. I think BoL and the Hyborian Age, may well make for an excellent campaign!
8. Maybe ZeFRS?
http://www.lulu.com/shop/the-zefrs-collective/zefrs/ebook/product-17399766.html
PDF is free. Hardcopy also available from Lulu. It's the clone of TSR's old Conan game system by David "Zeb" Cook. All REH Intellectual Property stripped out, leaving what many consider to be a fine Swords & Sorcery RPG.
9. Dungeonslayers! Available for free download:
http://www.dungeonslayers.com/
10. Warrior, Rogue & Mage - The name of the game and of a Character's three stats. Also Free:
http://www.stargazergames.eu/games/warrior-rogue-mage/
11. And Mazes & Minotaurs! All free with tons of support!
http://mazesandminotaurs.free.fr/