Updated with links!.
Rob Conley's Blackmarsh and Scourge of the Demon Wolf.
Chris Gonnerman's Morgansfort: The Western Lands Campaign. Also available as a free PDF.
J.D. Neal's The Chaotic Caves. Also available as a free PDF.
Gavin Norman's Theorems & Thaumaturgy.
Tim "Sniderman" Snider's Deviant Database for Mutant Future and other post-apocalyptic games.
Trey Causey's Weird Adventures.
New Big Dragon's D30 DM Companion.
Greg Gillispie's Barrowmaze (the original!).
Michael Curtis' Stonehell Dungeon.
RC Pinnell's BB1 - The Pearl of Pirate Cove. Also available in PDF.
The Warlock's Home Brew's The Outpost on the Edge of the Far Reaches.
Pacesetter Games & Simulations' I1 - Grave of the Green Flame, I2 - Beyond the Black Wall, and Q3 - Death on Signal Island.
John M. Stater's Mystery Men!, superhero goodness in an old-school RPG.