Portal 2 features a number of achievements that are related to “hidden” areas:
- Door Prize
- Final Transmission
- Ship Overboard
- Portrait of a Lady
- Party of Three
How can I unlock them?
Most of the hidden areas I can think of off the top of my head are achievement related (so the below might count as spoilers to some folks).
Specificially, the areas associated with Final Transmission, Ship Overboard, and Portrait of a Lady.
Portrait of a Lady
Beyond that, I know of two easter-egg rooms, the first holds the Turret Quartet, and the second is the secret developer commentary room (with the beeping screeches that, when translated, talks about the Devs involvement in the ARGs).