I do so love these ingenious little Flash platform games, each taking a novel concept and executing it so well. Shift is an excellent example.
A simple platform set-up - single screens, with keys to acquire and a door to reach. However, getting there requires being negative. And positive. That's a joke there. By hitting Shift, you flip upside down, into the other colour's dimension. The screen rotates, and now you can progress. It's muddling at first, but worryingly quickly you can adjust to thinking, "In order to be up there, I'll need to go down here."
It's short, but a fun old time. And it's very obviously inspired by Portal, directly referencing the game a number of times. Good luck getting onto that high score table, by the way. Perhaps it would have done better to leave these references out, as it's a neat enough idea to not need to compare itself to puzzle gaming's current Mother.
Many thanks to Martin for the tip-off. Get thee to Shift.