If the Flash version of Portal was just a tad too hi-fi for your discriminating needs, perhaps the Atari 2600 "demake" of the title will do. From The Independent Gaming Source's Demakes contest, in which developers were tasked with remaking current titles on old platforms — Shadow of the Colossus for the TRS-80 Model III, for example — comes Super 3D Portals 6.
The 8-bit re-imagining is playable on the Atari VCS emulator of your choice, but the author recommends Stella. It's also free. We're firing up our copy of Stella on the Kotaku Towers West PC to partake right now.
SUPER 3D PORTALS 6 - FINISHED [TIGsource Forums]