Like 29 other souls in the old-school gaming community, I recently received my "white box" of Jason Vey's Spellcraft & Swordplay. S&S is a reimagining of OD&D if, instead of fleshing out the "Alternate Combat Matrix," the creators had stuck through with a unified Chainmail-style combat system. Now, being a guy who worked out a matrix for weapon classes, I think that's pretty nifty, although I don't think I'd go with its specific interpretation.
The idea in S&S that I think is really remarkable is rolling for spells. Here's how it works: when a wizard tries to cast his spell, he rolls 2d6. It has three possible results: "Immediate," "Delayed," and failure. A failure indicates that the spell fizzles, except on a "2," when it's forgotten but still useable. Immediate means the spell goes off that round, Delayed means that it goes off the next round. The neat thing is that Immediate and Delayed results don't involve forgetting the spell until the next day. It's a very cool way to run wizards, especially so that first-level ones aren't necessarily one-shot ponies and high-level characters aren't indominable. It's a cool enough idea that I think it's worth adapting.
Of course, the S&S spell failure rates are a little high at low level, and I think it may need a broader "works but you forget it" option, as well as the possibility of an actual failure (and attendant "spell failure chart," natch). But when I finally get a miscellany written (see Jeff Rients's excellent Miscellanium of Cinder for an example of awesome in a can, will write more about this once I get it read) something inspired by Vey's spellcasting rules will probably work their way into it.