A homebrew programmer has cooked up a version of Portal for the Nintendo DS, titled "StillAliveDS." Naturally, this game plus an editor to create your own levels is free and available for download.
You move about the map, aiming a portal gun with your stylus to finish the level and grab pieces of cake along the way (naturally.) DS Fanboy raved about the game, conceding it lacks the graphics of the original but "it definitely has the feel and charm." Hell, I love the look on its own. Has a kind of Cartoon Networky mien about it. I just want to poke that guy in his tummy with the stylus, Pillsbury Doughboy style ...
Eh, er, OK, back to news. You can download it from the programmer T4ils, and if you need a guide on how to homebrew for the DS, DS Fanboy has that as well. There's also a screenshot gallery on DS Fanboy.
StillAliveDS: Portable 2D Portal for DS [DS Fanboy]