L-Echo is a free and open source clone of the SCEJ game Echochrome (Mugen Kairou in Japan) (PSP/PS3). It was written for Linux but has been ported to Windows, Mac OS X (10.4+) (NOT PPC64) and the Nintendo DS (the Nintendo DS port is named N-Echo and lives in the same source tree).
Notice: As of 3-20-09, I am no longer developing this. This is a failure on my part, because of my bad coding practices. I can no longer maintain and add to my code, so this project is becoming a huge time sink. I'm sorry for those who wanted a full echochrome clone; I guess you'll have to wait for a good PSP emulator, or another programmer who is willing to write another echochrome clone (I'm happy to teach the concepts to anyone who wants to take up the challenge).
Update 7-30-09, it occurred to me (in the process of preparing the last N-Echo release for a homebrew contest) that I never finished the doc on how to create stages. That's finally fixed.