Aperture Science, "Know Your..." Series of Safety Posters
"True Love Never Lies" by jimiyo (t-shirt available at RedBubble)
"What If Chell And Gordon Met?" by ~Lintufriikki
"Tiger Electronics' Portal 2" by zero-lives
"Genetic Lifeform and Disc Operating System Enrichment Centre Safety Handbook" by A.J. Hateley
"Chell Nouveau" by MeganLara (t-shirt available at RedBubble)
"Portal 2 Cubees" by Cubeecraft (full Valve series available here)
I've been playing a ton of Portal 2 lately. I'm nearly finished the single-player campaign and then I'll try tackling the multiplayer, but I have to say - there's so much pure awesomesauce crammed into the single-player story-line that I wish it would never end. Almost anything I say about Portal 2 would be a spoiler, so instead I'll just throw out a bunch of phrases, out-of-context, to express my enthusiasm: Wheatley! Reboot! Cave Johnson! Gel! Potato battery! 1982!
One thing I am able to confidently ruin for everybody is that The National's song recorded for Portal 2, Exile Vilify, is fantastic, and is worked into the game in a fairly non-essential (in that if you miss it, you aren't overlooking anything critical to the game itself) but well-implemented manner. Or you can just stream it below.