In The Groove for PS2 
(mirror from http://www.gamespot.com/ps2/puzzle/inthegroove/hints.html)

Unlock Everything
At the title menu, press:

Cheat 	Effect
Up,Right,Up,Right,Left,Down,Left,Down,Up,Right,Down,Left,Up,Left,Down,Right 	Unlock all 76 songs.

Force "Good" Game Developer Ending
Inputting this code at the main selection screen (Dance Mode, Battle Mode, Marathon Mode, etc.) will allow you to view the developer ending after playing a round of play, without getting at least an S+ average on your songs or marathon course. A few sparkles on screen will confirm correct code entry.

Cheat 	Effect
View Developer Ending After Playing
Up, Up, Down, Down, Left, Right, Left, Right


In the Groove Unlockables
You can unlock songs, courses, and modifiers by playing songs in the game, in any mode. Every 10 songs you play, you will unlock something.

Unlockable 	How to Unlock
Xuxa 	Play 190 songs
Wake Up (from ITG2) 	Play 200 songs
Incognito (from ITG2) 	Play 180 songs
Pandemonium 	Play 170 songs
The Legend [Marathon Course] 	Play 160 songs
Infection 	Play 150 songs
DJ Party 	Play 130 songs
Tribal Style (from ITG2) 	Play 140 songs
Funk Factory (from ITG2) 	Play 120 songs
Don't Promise Me ~Happiness Comes Mix~ 	Play 110 songs
Disconnected -Mobius Mix- 	Play 90 songs
Liquid Moon (from ITG & ITG2) 	Play 100 songs
Beat [Modifier] 	Play 80 songs
Bubble Dancer 	Play 70 songs
Anubis 	Play 50 songs
Outer World [Marathon Course] 	Play 60 songs
Bumpy [Modifier] 	Play 40 songs
Tell 	Play 30 songs
Bounce [Marathon Course] 	Play 20 songs
Disconnected -Hyper Mix- 	Play 10 songs

In the Groove Glitches
Avoid being locked into a song by Chance
Normally, when you pick Chance, you are locked into the song it chooses. However, if you select the song and enter the settings menu, then back out with Triangle, you are returned to the song wheel and are no longer locked into the song chosen by Chance.