If you liked Portal, you might want to check out the Quantum Conundrum demo that's been released on Steam. The game was designed by Kim Swift, who was also the lead designer on Valve's popular first-person puzzler. Quantum Conundrum follows a similar theme but looks much more whimsical. Here's the official trailer:
Instead of solving puzzles by shooting portals, players can shift the world through four dimensions. In the fluffy and heavy dimensions, objects are lighter and heavier than normal. A slow dimension offers effective bullet-time, while the reverse-gravity dimension is exactly what you'd expect.
Although the demo came out earlier this week, the full game has been available since late June. It's already garnered a Metacritic score of 78 with a 7.5 user rating. Don't ask me why the critic score is out of 100 but the user score maxes out at 10.
The full version of Quantum Conundrum costs only $15, which isn't much more than the price of a premium ticket to the new Spider Man movie. If you're curious, the demo is completely free.