Those who've seen my Orange Box Years Later video might remember this, but weirdly, Portal Still Alive isn't just a straight individual copy of the original game sold separately from Valve's iconic bundle, but a version of the game which had it's own Campaign with 14 original levels.
The strange thing is while these levels are "Advanced Chambers" they aren't made as if you're a Portal veteran familiar wanting to start the volume at eleven, but like Portal's original campaign doesn't exist, and that's because Portal didn't when these maps were originally made.
In-fact, the levels themselves are an adaptation of Portal: The Flash Version made by the same developer, and were so well received that Valve themselves hired him to enhance them further in Portal: Still Alive for XBLA. This Mod for PC takes those levels, and gives them level transitions that didn't previously exist, resulting in essentially Portal: Again.
Because this is essentially an original campaign, it's still testing the player basics of the game in early stages, and steadily ramping up the challenge until eventually building up to a climax taking you through the underbelly of Aperture Science's testing facilities.
It's one of the rare occasions where you essentially get to play a game for the first time all over again, and as somebody whose fantasy is ten Hollywood director's adapting the same script, Portal: Still Alive is something that I think about to this day.
Oh, and it's free on PC.
https://www.moddb.com/mods/portal-still-alive-pc