If you like a challenge with your PC games, Four Horsemen can give it to you. It doesn't hurt that the graphics and concept are quite cute, despite the underlying apocalyptic theme.
Once Pokki has been installed, you can run Four Horsemen in a matter of seconds. The object of the game is simple - just drop colored balls (called "plagues" for game purposes!) onto one of four squares to connect balls of similar colors. Once you've managed to entirely fill one box with the same colored balls, you'll get your points and that box will reset.
Keep an eye up at the top of your screen though. Each ball you drop is one less you have to play with. You earn extra balls by filling a box entirely with one plague color, and that will keep you going as you level up and the game gets harder.
If you're confused, there's a helpful tutorial built right into Four Horsemen. Even with it though, Four Horsemen can be a little confusing at first. Once the game begins to click with you though, it can provide challenging entertainment for just about as long as you keep collecting balls.
Four Horsemen is an addictive game once you learn how to play it, and the cute but simple graphics really complement gameplay.