Not to brag, guys, but over the last few years, I've collected a pretty impressive library of old school RPG products. Most of my focus has been on Classic D&D (OD&D, BX, BECM, etc) and some related AD&D stuff, but I've come across plenty of interesting OSR stuff too.
My taste in modern-retro OSR products leans toward the really creative and innovative; games like Lamentations of the Flame Princess , Carcosa , Adventurer Conqueror King (ACKS) , and Dungeon Crawl Classics . Although none of these really does it for me in whole, tempting me to give up BECM D&D entirely, each offers some new and unique material and approaches to the old school D&D system that can make great additions to an existing game.
Now, I have nothing against the more straightforward "retro-clones" like Swords & Wizardry , Labyrinth Lord , Dark Dungeons and others. I love these for keeping the old rules alive for new players, as well as showing off the various ways the rules can be tweaked this way or that to customize the game engine to any particular taste.
The thing is, there is/was far more to old school gaming than just D&D! Some of the other offerings of the late 70s and early 80s were essentially custom D&D builds tailored to the author's campaign and setting, Arduin was a good example of this. Other games, like Tunnels & Trolls, Runequest, The Fantasy Trip and many others, started from scratch, borrowing ideas from other games of the time but creating all new rule sets.
All these games, D&D "knockoff" or otherwise (though, to be fair, some elitists and originalists might call all those games D&D knockoffs) are ripe for the picking! Need interesting new monsters, magic and treasures? Just crack open an old school rpg and have a look, you're certain to find something weird (and sometimes wacky) that D&D never thought of.
We don't have to limit ourselves to OSR games or more modern versions of D&D (and their OGL and GSL knockoffs), nor be forced to do all the work ourselves with homebrew material. Even if you enjoy doing homebrew, some fresh inspiration now and then is never a bad thing.
What are your experiences and opinions of the non-D&D old school games?