There is Dungeon World - the free version does not have pretty art and fancy layout, but it has the full text and is functional. It is a rules-lite retroclone of D&D which uses the tropes of D&D as its roles. Each class has a set of “moves” , and when you use one, you roll 2d6 + a stat. If you score 10 or above, you get what you want. If it is lower but above 7, you suffer a side effect.
For example, the Bard has a move “Turn it up to 11”. If you roll Cha + 2d6 and get 10+, you execute it perfectly. If you roll less than 10 but above 7, it goes out of control and the GM picks 1 additional target.
There is also Old School Hack, another retroclone which is good for a pick up game. You may also want to check out Warrior, Rogue & Mage - it’s a simple game with just three stats – er, which are Warrior, Rogue and Mage. So if you want to smash something, you roll 2d6 + Warrior. To do something sneaky, use 2d6 + Rogue. And so on.
If you prefer something closer to d20, there is also Mircolite20, which is somewhat similar to the WR&M game above. You have three stats - Strength, Intelligence and Dexterity, and 4 classes - Fighter, Rogue, Cleric and Mage. There’s a skill associated with each class - Physical for Fighter, Subterfudge for Rogue, Communication for Cleric and Knowledge for Wizards. There are many expansions to this simple system to include clerics, paladins, rangers etc.
You may also want to check out Dungeonslayers. It’s a rather medium crunch RPG, and it’s already in its 4th edition.
The last free, good RPG I can of at the top of my mind is also OpenQuest, which is OGL version of RuneQuest. Standard D&D stats (it drops Charisma for Power and adds in a Size attribute), but it uses percentile to represent skills.
Oh, if you don’t mind checking out drafts, there is also Barbarian of Lemuria. Rules-lite and lots of fun, especially the magic rules. It’s more Sword & Sorcery than high fantasy though. The commercial PDF can be bought off cheaply, the draft can be found through Google I think (it was a submission to a 24 hours RPG contest).