REQUIRES PORTAL 2. A companion mod to the 7-part interview series at Rock Paper Shotgun that focuses on discussing level design and process. You could also call it an "interview mod," I guess. Or maybe it's one of those dreaded art mods? I'll let you decide how to label it, but given the choice, talking about the game rarely matters as much as playing it. So just play it. (This mod was sponsored by Rock Paper Shotgun; built and compiled by Robert Yang; contributors are Dan Pinchbeck, Jack Monahan, Magnar Jenssen, Brendon Chung, Davey Wreden, Ed Key, and Richard Perrin.)
OSX users: 1) FIRST DOWNLOAD THE MOD and install it into something like "~/Library/Application Support/Steam/SteamApps/Common/Portal 2" (the folder is hidden by default on OSX Lion, so go to Finder >> Go (menu bar) >> Go to Folder)... 2) Restart Steam, AND THEN 3) download and use this .app that acts as a shortcut... If you want to do it the manual (and less obtrusive) way via Terminal and making a symlink, read more...
There's another way to do it, if you don't like that way...
On OSX, the problem is that Steam doesn't execute (though it properly scans, or something) from /sourcemods/ or something, so you have to make a symlink to the Portal 2 directory.
1) Keep the extracted /levelwithme/ mod in Steam/SteamApps/sourcemods/ instead.
2) In Terminal, execute:
~/Library/"Application Support/Steam"/steamapps/common/"portal 2"/ ln -s ../../sourcemods/levelwithme
3) Restart Steam.
4) Then in the Steam games list, right-click on "Level With Me" >> Properties >> Set Launch Options.
5) Set launch options to -game levelwithme press OK and now you should be able to start the mod from Steam while keeping your core Portal 2 folder uncluttered.