>12TB of unreleased game files from various developers were stolen back in 2013 from valve servers and released recently
>twelve (12) terabytes !!

Some findings:
>Mirror's Edge prototypes / beta, earliest builds from early 2007
>Spore beta with earliest files going back to 2006 (!)
>COD4: MW, WaW betas
>VTMB prototypes and beta builds
>Warhammer 40K: Dawn of War prototype / in-development assets
>Batman: Arkham Asylum beta content
>Dragon Age: Origins near complete development cycle
>Portal 2 2009 build
https://steam2.download/

It's probably not source code but some will still be based.
There might be half life episode 3 stuff in there