There are 16 Secret Eggs to be found in the game, one for each level of the game. Whenever you find a secret egg, it will unlock one of the following items. And the unlockables are unlocked in the following order, regardless of which egg you've found first.
Effect - Code
Bonus Level: Throne Room Find Secret Egg # 16
Commentary - Art Direction Find Secret Egg # 03
Commentary - Birth of a Game Find Secret Egg # 04
Commentary - Design Find Secret Egg # 02
Commentary - Edward John Speleers Find Secret Egg # 01
Commentary - Evolution of Games Find Secret Egg # 14
Commentary - Garrett Hedlund Find Secret Egg # 13
Commentary - Orchestra Find Secret Egg # 06
Commentary - Robert Carlyle Find Secret Egg # 11
Commentary - Saphira Find Secret Egg # 07
Commentary - Sienna Guillory Find Secret Egg # 08
Commentary - Sound Effects Find Secret Egg # 12
Concept Art - Beor & Varden Find Secret Egg # 15
Concept Art - Characters Find Secret Egg # 09
Concept Art - Gil’ead Find Secret Egg # 10
Concept Art - Spine, Carvahall & Daret Find Secret Egg # 05