Alternate Reality Game puzzle design - Adam Foster of Valve talks about the design of the Portal 2 ARG, (previously) and do's and don'ts for future ARGs.
25 extra minutes of Cave Johnson - taken Portal 2's Perpetual Testing Initiative DLC.
No GLaDOS. No Chell. No portals. Set in the 1980s. Competitive multiplayer. Multiple endings. The Portal 2 That Could Have Been.