Give Up, Robot is an awesome platformer -- Time Waster
I literally had to tear myself away from this one just to write about it: Give Up, Robot is so much fun.
It's a low-fi platformer with very engaging gameplay. You're a robot (I know, you never would've guessed that on your own) and you can run, jump, and hook onto things with your grapple (using Z to shoot it out). Once you've attached the grapple to anything, you can swing yourself using the left and right keys, and extend or shorten the rope using up/down. As soon as you let go of the Z key, the grapple disconnects.
Translated into action, this means you swing around like crazy and then propel yourself across the level like the robo-monkey you've always wanted to be.
There are all sorts of dangers along the way; there are bricks that move around, and bricks you can't stand on. There are also propellors that shred you to bits if you touch them, but you can grapple onto them, and then they start rotating you around like crazy. You then let go at just the right time, and launch yourself to wherever it is you're trying to get.
Each level is exactly one screen long – at least all the way to level 17, which is when I managed to stop playing long enough to write about it. Loads of fun!