Rock, Paper, Shotgun points out a video showing Portal, redone with ASCII graphics. It's still in development, but appears to be quite far along. Its creator, Cymon, says on his website, "I have Windows XP, so all binaries will by default be for Windows. But I will also be including the source code with the distribution and am doing my best to write it cross-platform compatible, so it should compile in Linux and Mac. I've had successful builds done in Linux." He also talks in detail about his design plans and ideas.