Let's start with everyones favorite game: Tetris!
Helltris
This version can be filed under the "impossible" category. It's based on the infamous xkcd comic strip.
Hatetris
This one might be possible, but it's insanely evil and will make you rage. I managed to get 2 lines done.
Tetoris
You got an hour or two? Might as well try this one!
First Person Tetris
This Tetris clone is absolutely possible but takes quite some time to get used to.
Now on to non-tetris games!
Athletics
Run 100 meters! Sounds simple? Not if you have to control your muscles manually. I stopped at 33 meters (without falling) since it took ages to get further :/
Dead Drunk
Can you make it home from the bar... ALIVE? Seriously, this game is awesome, I finished it in about half an hour but was close to throwing my computer out of the window at times.
(Warning, contains alcohol and blood... and music by Lady Gaga)
Yeah, that's it for now - Feel free to post your experiences with these games and/or post links to equally hard flash games.