Various cheats

Metal Sheen:
     Hold  A  +  Triangle  after  selecting a 2-player level to have a
"metallic" skin covering your team.

Unlockables

Super Hard Difficulty: Beat the game with 141 Emblems and all A ratings
Metallic Characters: Collect all 141 emblems
Last Story: Collect all 7 Chaos Emeralds and finish all stories 

Unlock Media

Team Chaotix Cinema     Complete Team Chaotix in Story Mode
Team Dark Cinema        Complete Team Dark in Story Mode
Team Rose Cinema        Complete Team Rose in Story Mode
Team Sonic Cinema       Complete Team Sonic in Story Mode
Sonic Heroes Song       Unlocked at start
Follow Me Song          Complete Story Mode as Team Rose
This Machine Song       Complete Story Mode with Team Dark
We Can Song             Complete Story Mode with Team Sonic
What I'm Made Of Song   Complete the last story

Unlock New 2P Modes

2P Action Race          Unlocked from start
2P Team Battle          Collect 20 emblems
2P Special Stage        Collect 40 emblems
2P Ring Race            Collect 60 emblems
2P Bobsled Race         Collect 80 emblems
2P Quick Race           Collect 100 emblems
2P Expert Race          Collect 120 emblems