Remember wishing for game X to arrive on system Y, yet it never did? Well you should check and see if it ever happened, because it just might have, and maybe recently recently. Because the homebrew development scene is always trying their best to make wishes come true.
As is the case with Aperture Science, which is basically Portal, but shrunken down for the DS. Though it also happens to contains a considerable amount more than the original game.
As Joystiq explains, it's the result of two members of GBATemp, perhaps the longest running and most well known corner of the web when it comes to doing stuff with one's DS (and PSP, and 3DS, plus more) that you're not supposed to do:
Looks quite impressive, to say the least, but there's more. Like an original story; it's about Doug Rattman, the scientist who wrote "the cake is a lie" and also saved Chell from certain doom between Portal 1 & 2.
Even though the game is incomplete, there's still a level editor! To give it a shot, and if you also have a flash cart to run it on, Aperture Science can be downloaded here.