ASCIIpOrtal

ASCIIpOrtal is a 2D ASCII version of Valve’s Portal. Run around, fire portals, jump through portals, dodge barrels, push switches and enjoy the company of your companion cubes... all in wonderful text format.

Interestingly (or not really) the character set used isn't ASCII - for instance the green player character (see Wikipedia). Also the game's author lists his inspirations as "Increpare’s Portile game, Super Serif Bros, and Valve’s Portal". So it's not ASCII, nor is it just Portal, but it is definitely awesome.