Recently I splurged and ordered all four One Page Dungeon Compendia from Shattered Pike Studio at RPGNow.com.
At a cost of $11-$12 a piece, with each book stuffed with between sixty and ninety adventures, the value is pretty incredible. Of course, you can always scratch around and find each of these scenarios plus more submissions that didn't make the book online ... but the softbound, full color collections are just nice.
For me, along with The Dungeon Alphabet, these are probably my best RPG purchase since I bought Scourge of the Slave Lords (A1-4 bound together) from Walden Books back in 1986.
Though, it's a bit of an oversimplification, I sort all published OSR materials into three groups: rules sets, supplements, and adventures.
Rules sets are fine, and many of them have lots of charm (it's fun to look over the fence at what houserules others have implemented), but apart from a gem or two (like the idea that Knock and Wizard Lock are reversed versions of the same spell, or White Star's super-simple ship-to-ship combat), these have a pretty low impact at my game table.
Supplements introduce new monsters, spells, items, and classes, but I've seldom felt much need for these since B/X is already so flexible. Want to play a pixie? Sure. Just stat it as a thief (climbing rolls stand in for aerial maneuvers) and get on with play.
That brings me to adventures--for me that's where the "good stuff" is--ideas that I can implement at my table. The problem with even many of the classics is that, within the 30-odd pages of a module, you have some great, imaginative encounters, a larger proportion of pretty standard ones that don't offer anything particularly new, and a few stinkers that I'd never inflict on my campaign. Any published adventure is sort of a crapshoot ... I never know quite what the proportions will be until I've dug through the each page.
For me, that's where the one-page format really shines. Because of the space restrictions, writers lead with their very best punch. Granted, sometimes the ideas are pretty standard stuff--there are some real stinkers in the One Page Dungeon Compendia, but I'm not banking all that much time (and less than 25 cents) per scenario. Overall though, I've found them to have a much higher ratio of clever, useable, and innovative material than is typical of the dozens and dozens of full length adventures that I've collected over the years.
Finally, I think that, as a product, these showcase the eager-to-share and do-it-yourself approaches that reflect the role-playing community at its best.
What's been your best role-playing purchase of recent years--the one that you couldn't help but gush over?
What made it so darned good? Was it ease of use / application at your table or did it scratch some other itch?