My return to active RPG Gaming was due in large part to Castles & Crusades. Heck, I played in a D&D campaign for the better part of two years via Fantasy Grounds 2. It was fun and certainly felt like D&D to me. In many ways, it was my introduction to the OSR before there even was an OSR movement.
I still buy much of the C&C stuff that is released, even if my current game of choice to run as a campaign is ACKS (and of course DCC RPG in shorter arcs). C&C Classic Monsters is the poor man's Tome of Horrors Complete if you ask me ;)
That being said, the true diamonds are Fields of Battle at $3.35 (though the mass combat rules can work in most OGL / OSR games), Engineering Castles at $2.10, Engineering Dungeons at $2.39, Arms and Armor at $3.00, Heart of Glass (adventure) $2.12 and the Aihrde Campaign Setting for $3.99 among others (a bunch of adventures are $2.10 each).
Not sure how long th sale is for, so strike while the iron is hot ;)
The Minotaur for Old-School Essentials
*Requirements:* Minimum STR 9, Minimum CON 9
*Prime Requisite:* STR and CON
*Hit Dice:* 1d10
*Maximum Level:* 8
*Armor:* Leather, including shield...