If I asked you what game on Kongregate is most like Portal (the real Portal, that is), your first guess might be Portal: The Flash Version. But, as I mentioned earlier, Portal: The Flash Version doesn't actually share all that much in common with its parent. In fact, it's SHIFT (or perhaps SHIFT 2, but we'll get there in due time) which is far closer to the spirit of Portal. It's got a simple but intruiging and creative concept at its core, it's very playful, and overall a very short game, so that you can play it in a single session but still feel that you would like to try more.
The basic concept behind SHIFT is very simple. At first, it looks like an ordinary platformer-type game with platforms and keys and deadly pits of spikes. However, the twist is simple, but very interesting -- when you find yourself stuck, you can press Shift, and suddenly the screen rotates 180 degrees and instead of a black figure on a white background standing on black floors, you're a white figure on a black background standing on white floors. With up and down now switched, it's a piece of cake to get to the high ledge you couldn't reach before. (Or maybe it isn't...)
The game teases you in much the same way that Portal does (indeed, the parallels between the timer and the cake are obvious, and the author makes no secret of where exactly he got his inspiration from), and the whole thing has a very light-hearted feel, which makes it an enjoyable experience to play.
That said, the game is somewhat limited, first simply by the fact that the size of the Flash screen means that the puzzles remain pretty small and hence nothing is too complicated, and second by the fact that there just aren't many levels in the game. (Also, the last level seems a bit misplaced in a game which otherwise is pretty cerebral.) So when you finish, you'll definitely be surprised at just how short the experience was. Still, it's much better to have a game which provides a quick, pleasant experience than one that overstays its welcome (Areas, I'm looking at you...).
The presentation is appropriately sparse -- the graphics are very simple black and white (the better to survive the inversion, I suppose), and there's no sound, though the background music is a nice touch. Like many other Kongregate games, there's no way to go back to specific levels, but since playing through the game is so quick, it doesn't take particularly long to get to any level anyway, so this isn't perhaps as much of a handicap as it might be in other games.
Anyway, there is a SHIFT 2 out, and a review for that coming up soon, so if the game does leave you with a thirst for more, you can get more! But if not, it's still a fun way to spend 20 minutes, and something that really shows the spirit of good game design at work.