Devil May Cry 3: Dante's Awakening

Kill 100 enemies during the credits to get a different ending screen.

Press R1 or L1 at the mission select screen to get different characters. Beat the game on normal.

In the 3rd mission there is a jukebox in the bar. If you slash it with your sword you'll hear a remixed version of "Lock and Load" (a song from the original DMC) for a short period of time.

Die twice in normal mode.

Hit L2 after firing a slow to reload weapon and flick it back to fire it again. It is slightly faster than waiting for the weapon to reload.

At the start menu hold L1 + L2 + R1 + R2 then rotate the Left Analog-stick. You will here a voice say "Devil May Cry and all modes, costumes, and galleries will be unlocked.

In each intro cutscene for each level, the game director included hidden "numbers" that correspond to the action mission level. The hidden numbers in the cutscenes for each level is listed:

Mission 1: The number "1" on the pizza box
Mission 2: The number "2" on the billboard at the end of the cutscene
Mission 3: The burning enemies form the number "3" at the end of the cutscene
Mission 4: The hanging banner/flag at the beginning of the cutscene is shaped as the number "4"
Mission 5: The blood on the wall at the end of the cutscene forms the number "5"
Mission 6: The explosion at the end of the cutscene leaves the number "6" on the ground
Mission 7: The blood stain on the wall reveals the number "7" at the end of the cutscene
Mission 8: The moon in the background has the number "8" engraved in it
Mission 9: The bullet at the end of the cutscene has "9mm" marked on it
Mission 10: The blood on the book at the end of the cutscene forms the number "10"
Mission 11: The blood on the ground marks the number "11"
Mission 12: At the end of the cutscene, the chain attached to the ceiling is holding the number "12"
Mission 13: At the end of the cutscene, the wall in the background has the number "13" engraved
Mission 14: The number "14" is on the side of the platform, in the pit
Mission 15: The branch at the end of the cutscene shapes the number "15"
Mission 16: The number "16" is lightly scratched on the ground. It's easier to see the number at the beginning of the mission
Mission 17: The top of the pillar at the end of the cutscene forms the number "17"
Mission 18: A paper containing the number "18" quickly flies by in the air
Mission 19: As the camera pans to Lady at the beginning of the cutscene, the number "19" is lightly marked on the ground
Mission 20: The number "20" is formed by the clouds in the background.

Beat Dante must die mode.

When loading, you can press either 'triangle' or 'square' button to slash or shoot respectively at the 'NOW LOADING' word.

A great place for orbs is the fifth secret mission found on stage 9. Go through the level untill you get to the area where you get the spiral. Go through the passageway underneath the waterfall and up through a small tunnel directly across from where you. The secret mission is marked by a few bright glowing crystals. Once inside you must destroy every breakable object within 40 seconds. The jukebox contains 500 red orbs, although there are many other things to smash as well. Since you can repeatedly pass/fail this mission and not encounter any enemies, you can repeat this process for lots of easy orbs.