I've offered this one up as a Free and Worth Every Penny entry for Mike to do - if he doesn't take it I surely will myself. But I want to tell you right now to go download Super Crate Box. Between this and Super Meat Boy (which is sadly not out yet on the PC but I've been playing on the 360) I have just about all the retro super-hard platforming I can handle, and I love it. I started out this evening playing a leisurely round of Civ V, but then Super Crate Box came along and it was all jumping and shooting all the time.
What the heck is it? Well, it looks like this:
...and the gameplay is exactly as insane as it appears. You're on a single-screen layout, not dissimilar to the original (pre-Super) Mario Bros arcade game. Enemies are spawning at the top and making their way to the bottom, also not unlike Mario Bros. What is unlike Mario Bros is, of course, the obsession with crates. They are your only scoring mechanism - the more you get, the better your score - but every crate forces a new random weapon on you, and it's frequently an unpleasant surprise which one you get. You need to manage the constant weapon switching and avoid enemies while collecting crates until you die, which will happen A LOT.
And that's it. But it's great and free. A longer writeup will come from me or Mike later, but for now, just go get it. You'll want a gamepad, I imagine. I certainly wouldn't have wanted to play it with a keyboard.