I have to give full disclosure for this one: I am actually in the "Thank You" credits for this one (right at the end, I'm "Paradigm"). I was a fan of it back when less than a thousand people knew it existed in ModDB. So if that at all worries you about my bias towards this, fear not, it's actually become fairly popular on it's own right (and was even reviewed by Game Informer), and I'm still openly critical of the things it could have done better. With that out of the way...
Tiny and Big: Grandpa's Leftovers will remind you of the time when game developers just took an idea and ran with it. It is an unconventional game looking purely to it's own designs rather than trying to fit in any particular genre. At it's most basic, it's a puzzle platformer, but with three core mechanics instead of just one gimmick.
You can cut almost anything with your laser, you can drag almost anything with your hook claw, and you can propel almost anything with a rocket. You can make your own bridges, platforms, or knock aside walls in your way to progress. With a custom made physics engine built for the game, it's almost a miniature sandbox of options. Despite a linear progression through the campaign, every level hides secrets and can be solved multiple ways. At one point near the game's end, I was able to just use my knowledge of the game to bypass a puzzle and it all continued to run properly.
Tiny & Big: Grandpa's Leftovers is the sort of game for people who really want a fresh experience, even if it's still got bugs and little slip ups due to the nature of it's freedom. Checkpoints can sometimes be unforgiving, your response time can be pushed to the limit in some boss fights, and the physics can get wonky if you do something -really- off the wall.
It's also a great game for indie music enthusiasts. One of the things you collect in the game is the game's excellent soundtrack, with some songs hidden in places most games wouldn't even let you access. Combined with a more meta-story about ancient gods you can only uncover through thorough exploration and some hilarious moments the developers have for certain playstyles, there's a hefty amount of replayability for collectors.
If any of this catches your interest, it's worth at least a try of the game's Steam demo. Considering the fairly low requirements for playing it on PC, Mac, and Linux, it's probably the second least stressful game on PC hardware on this list, so even primarily console gamers can give it a shot.