Bundle of Holding - Castles & Crusades - Starting at 12 Bucks
I like Castles & Crusades. I like it a lot. I've referred to it as the Rosetta Stone of the OSR in the past and I still see it that way. You can pretty much grab an adventure from any edition (can't speak for 4th) and run it in C&C pretty much as is.
On a side note, I've also referred to C&C's Classic Monsters as the frugal DM's Tome of Horrors Complete. Much of the same source material, compatible with most editions and much more affordable.
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