There are a lot of options available. Each has pros and cons.
Castles and Crusades might be just the thing you're looking for. Check out all the reviews (mine included) here:
http://www.enworld.org/reviews.php?do=product&productid=119800
Yes, it does have some typos, but I don't think they are huge. The bigger annoyance is the order in which the classes appear (Apparently in prime attribute order, not alphabetical order). A revision is coming out this winter. C&C is good because it's compatible with pretty much every flavor of D&D. It's not so great because it tends to be a "black box" with regards to the classes. Each class has a separate XP table, and there's no rules for knowing how that chart is based (it's not arbitrary), but how each chart was built, isn't included. So, if you want to modify a class, you have to just guess on how to adjust the XP chart. If they opened that up, it would be a lot more useful. Also, there are no feats, and no prestige classes, and the rules for multi-class and dual class are a bit shakey. And there are no rules for such in the core rules. You have to get Castle Zagyg for the latter, and the Troll Lord forums for the former. But, if you want a light, fast game, that's compatible with earlier editions, you might really like it.
HARP is nice because the system is really well-grounded, and internally balanced. HARP is expertly written, and the quality is really superb. The combat is really deadly, and it scales really well. So a 20th level HARP character isn't all that powerful compared to a d20 20th level character. Players have a lot of character creation flexibility. There are a lot of variant rules available for HARP as well. In fact, I just asked Rasyr for a d20-like combat mechanic, and he delivered in spades in Harper's Bazaar volume 4, which was just released. The downside to HARP is that it is less compatible with D&D modules and such, so you'd have to do a fair amount of conversion if you're going to run some 1st edition modules, for example. But, you can check out HARP for free, so there's really no excuse not to. Go here:
http://www.harphq.com/free_downloads/3000L_HarpLite.pdf
Grim Tales is another option. Grim Tales is based on d20 modern. It's an open-ended system designed to run any genre of game, from fantasy to sci-fi. I admit that I haven't read very much of this book (I just haven't had the time), but from what I've seen it looks really good, and it gets a lot of praise on these boards, which says something. Grim Tales has a lot of options for making it more grim or more fantastical, which is really nice. It's a great system for low-magic style campaigns, which is the purpose of Grim Tales.
Yet another choice is True20, which is the core system found in the Blue Rose campaign Setting. I admit that I don't know much about True20 either, but the pdf is available for about $12, I think. True20 is based on feats. The classes are a bit more general, and you tailor them with feats. There is also a "damage save" mechanic which replaces hit points. It's quite unique, and gets a lot of praise here, but I've heard that it's a little hard converting older modules for it since it is quite a bit different.
There are others that people might recommend. Conan, Iron Heroes, or Arcana Unearthed are all good systems.