Although not old school in the D&D sense, Fighting Fantasy is pretty old school and the Advanced Fighting Fantasy rules have been released in PDF. Kinda pricey at $14
Also have a Sci-Fi version of the rules on Kickstarter. I liked that one starship FF book a lot.
While I hate C&C and don't think it's particularly old school, Luke Gygax apparently did some work on the Undercaverns of Gaxmoor
FRS1 The Dalelands - 2e Sourcebook for the part of the Forgotten Realms into NASCAR. $4.99, 64 pages
World Builder's Guidebook - Remarkably useful book for designing a campaign setting. Nothing mind blowing, but lots of charts and tables. $4.99 and 96 pages.
AdventuresOne Page Dungeon Compendium 2015 - While I think the format really kinda sucks for producing usable adventures (reminds me of haikus, too clever for its own good), I seem to be in the minority. PWYW
They Met in a Tavern - PWYW one-shot adventure with pregens. 18 pages
Motley Jerks - Free batch of strangely constructed 1e characters.
SL&vl #1 - Seems to be some OSR fanzine in Finnish. 43 pages, PWYW. Strangely, despite clearly being foreign, the suggested price is a $1