The basic version of the newest edition of Hackmaster can be downloaded for free. I figured for that price, it's certainly worth a look. Looks interesting, but VERY different from what I'm used to. The general concept is the same as 1st edition D&D, but the mechanics are VERY different. It's a little crunch-heavy for my tastes, but looks like a good take on a simulationist design philosophy (certainly WAY better than 3rd edition D&D). Seems like a very gritty and dangerous game.
The biggest thing that strikes me as unique is that there are no turns. The GM does a count-up in 1 second intervals, and you can act whenever you want (within the scope of the mechanics). You roll initiative and you want to roll low because that's what second you're allowed to first act. You move 5 ft per second (or half that for Dwarves and Halflings), and you have weapon speeds. The way these work is that after your initial attack, you can't attack for another X number of seconds, where X equals your weapon speed. For example, a longsword has a speed of 10 seconds, a battle axe is 12, and a dagger is 7. GM bookkeeping strikes me as being potentially difficult (perhaps even an outright headache!), but the advantage of this system is that all players are engaged all the time instead of waiting for their "turn."
Armor makes it easier to be hit (because you're slower), but soaks damage. Shields make it harder to hit, and can soak damage as well, but they can be shattered (by high-damage attacks, often after making an opposed roll to confirm the shatter). You make a defense roll every time you're attacked, and compare that to the attack roll result. Rumor has it that combat is still fast despite all of this dice rolling. If you take a certain amount of damage (equal to or greater than your threshold of pain) then you must make a trauma check, and if you fail you're down on the ground, unable to act for a certain number of seconds.
Anyways, I've only skimmed through it briefly and those are the major things I've picked up. I'm not necessarily sure that I'd want to run it regularly, but I'd be willing to try it out.