I have to admit this may not be a game for everyone. You basically burn stuff, and different things react differently. It's not difficult, and some people might not enjoy it because it poses little challenge. But for me, this is an absolutely fantabulously awesome experience.

Little Inferno is not a game of skill. It's a game of discovery. So if you like discovering, I highly recommend it to get it right this instant.

Garry's Mod is a great 3D sandbox. You can make a tower of guys handstanding of each other and take a screenshot. Or you can make a video of a leg being projected from a makeshift see-saw. Or you can just spawn zombies and kill them all.
There are heaps of tools and heaps of models to help you do this. And if you haven't got enough models, you can download even more.
Spawn anything. Move anything. Arrange anything.

Have fun.

This is a pretty sweet game. For a free MMO, it's refreshingly unique in both plot and playing style. It's a nice mix of Zelda-style gameplay, while still keeping the features that make it an MMO, like a guilds and trading.
Its only vice is the repetitive gameplay, which is basically just dungeoneering. However, there is an extremely wide range of (seemingly hand-made) levels, and the co-op aspect makes it even better. What's more, it doesn't matter who gets which materials, since you all share the loot!