I'll admit, I only downloaded Whoopservatory because of the silly name. I couldn't resist. But I'm entirely happy I did, as it's an excellent way to spend the 20-ish minutes it takes to explore the abandoned laboratory of Dr Magnusson, Half-Life 2 Ep2's grumpy counter-part to Dr Kleiner. Magnusson has made, ooh, bad things happen at an isolated observatory and Freeman has been sent to show them Crime (left fist) and Punishment (right fist). Fittingly, for someone so inept at human interaction as Magnusson, the central twist of this map is that it's co-operative, but with yourself.
Hence "Play With Yourself" you mucky minded sods. The Half-Life 2 wrapper here makes it easy enough to get into. Freeman shows up in his trademark jeep but without his trademark GG: only room for one gimmick in this short mod. After searching the house from the basement up, fighting off a couple of zombies, you're let into a room to play with a new toy.
The co-op works by recording your movements and playing them back, albeit as a glowing electrical ball and not as Freeman, in a seven second loop. The ball powers electrical equipment for you to use to escape the haunted feeling house. This is how it works.
There's naught but a couple of puzzles here, working how the ball can affect a couple of locked door scenarios, as well as a few neat moments of shooting, but it's the sort of short diversion you'll be glad you played. Install it for the next time you have exactly 20 minutes to kill and realise there's a scope for this sort of thing. I do wish more games could fit a short, twisty, story-led blast into 20 minute chunks.
As it happens, RPS chatted to the designer Magnar Jennsen as part of the Level With Me Series, so if you have more than 20 minutes you might want to see what he has to say about the making of this and his other works.