A sequel to Valve’s Portal has been announced – but not by Valve. Portal Prelude comes from a three-man dev team that have been working on an unofficial prequel to The Orange Box hit since January. It’s a bigger game that harks back to a pre-GlaDOS era, it’s coming very soon (end of September/October time) and best of all, it’s completely free! All you need to play it is Steam and the original Portal game. Check it out here.