The PSXSDK is an open source software development kit for the Sony PlayStation (PS1) based on freely available resources and resources written from scratch. FOR NOW: Use the WIP versions in the Download section. 0.1 is too buggy to be used for anything. (SVN not maintained right now.. no real need to)

Also checkout PSXBrew, free homebrew for the PlayStation! http://code.google.com/p/psxbrew

Official PSXSDK forum: http://nextvolume.net84.net/forum/index.php?board=3.0