Infinite Orbs:
For any Underground Orb mission, if you grab the Orb at exactly 0:00, you'll get a "Mission Failed" and also the 3 Orbs you won. If you choose to try again, you can do this over and over and keep obtaining 3 more Orbs.
Dark Jak vs Final Boss:
After the cutscene before the final battle, hold R1 and you'll transform (or stay as) Dark Jak, regardless of Dark Eco meter.
Unlockables:
Alt. Scrap Book - Collect 200 orbs
Big Head - Collect 30 orbs
Hero Mode - Collect 200 orbs
Invunerability - Collect 175 orbs
Jak's Goatee - Collect 5 orbs
Level Select - Collect 145 orbs
Mirror World - Collect 15 orbs
Peace Maker Gun Course - Collect 105 orbs
Reverse Races - Collect 135 orbs
Scene Player Act 1 - Collect 65 orbs
Scene Player Act 2 - Collect 95 orbs
Scene Player Act 3 - Collect 125 orbs
Scrap Book - Collect 55 orbs
Small Head - Collect 45 orbs
Unlimited Ammo - Collect 155 orbs
Unlimited Dark Jak - Collect 165 orbs
Vulcan Fury - Collect 75 orbs