Sounds

• http://www.findsounds.com/
• http://freesound.iua.upf.edu/whatIsFreesound.php
• game songs

Software

• http://www.renoise.com/
  • vst's: http://www.kvraudio.com/get.php
  • vst modules package: http://mda.smartelectronix.com/
  • songs: http://modarchive.org/
  • samples: http://www.sampleswap.org/
• audacity a free sound editor
• sfxr a lightweight sound generator
  • cfxr (osx version of sfxr)
• audio format converter for osx xld

never checked myself

• http://csounds.com/ i'm slowly used to find this, if i search for sound applications
• http://www.buzzmachines.com/ (windows only)
  • http://code.google.com/p/aldrin-sequencer/ (not tried)
  • http://code.google.com/p/armstrong/ (lib of aldrin)
• http://ardour.org/ sound editing for linux & osx
• http://www.drpetter.se/project_musagi.html (not tried myself, windows only)
• fl studio is apparently one of the best known softwares, also windows only...