There’s a feeling one gets when finishing Portal of “Wow.” And shortly after that, “Wait. It’s done?” And yes, it is. The game is excellent, and some actually praise its shortness, but I felt it could have used a bit longer of a mid-section; after you learn the controls but before things get out of hand there was room for a few more puzzles. Fortunately, third party developers (that is to say homebrew mapmakers) have taken up the task.
You may have already seen the Portal: The Flash Version map pack, which was excellent (I’m stuck, I’m ashamed to say). There wasn’t much context for that one, but this new thing, Portal: Prelude, is intended to be a full separate game with a separate storyline and an equivalent amount of gameplay. It should be complete “around the end of September.” Better lube up them neurons again.