Players that earned a golden potato from the Portal 2 ARG also earned every game Valve has ever made.
Valve's Portal 2 alternate reality game (ARG) was an amazing structure of hidden messages that led to interesting concept art, indie games hacked with Portal levels, and eventually the power for fans to get the game released early. Completing certain actions related to the ARG, such as specific tasks found in the 13-game Steam Potato Sack bundle, awarded users with potatoes in their Steam profiles. Accumulating enough imaginary potatoes has had a significant real world consequence, as those that earned a "golden potato" have been awarded with the Valve Complete Pack.
Destructoid confirmed that the Valve Complete Pack was awarded for holding a golden potato, a special spud earned in the ARG by acquiring all 36 possible potatoes. Some of these were much tougher to get than others. For example, in Super Meat Boy, players picked up a potato for dying 11 times on world 4-19. However, in Cogs, players had to figure out a strange puzzle involving glyphs that seemed impossible unless you'd been following the mysteries of the ARG.
To anyone complaining that Valve was trying to drain fans of money with the ARG and the Potato Sack, what do you think now? This is an amazing reward, and just icing on the cake for fans that probably had a ton of fun discovering the secrets of GLaDOS and Portal 2. Not to spoil it, but the whole potato thing is eventually explained in Portal 2, just in case you thought Valve lost its mind.