Fabian –
If you have the CS4 Production Premium suite, or Soundbooth, you have access to a pretty large (and free) library of music and sound effects. They can be downloaded from the Soundbooth interface – there’s a tab for Resources which will get you to them.
There are some free (albeit fairly low sampling rate) sounds out there on the web – FindSounds.com comes to mind – I’ve used many of them, and tailored a bunch of their sounds in the past – pretty good stuff.
If you’ve got audio editing software, or music software, there’s a wealth of free stuff as well. Search on SoundFonts, and you’ll find a ton of free sound patches to be used in VST plugins for the various music/audio editing packages (such as Sonar and Cubase). If you’re a musician, the average 300 dollar Yamaha synth has a huge library that can be recorded and customized to any computer with a sound card – that way you can really get original. Good luck!
Joe Bourke
B&S Exhibits and Multimedia