Warning, you could easily spend weeks just D/Ling and trying these things out, don't forget about making music!
Web Places: Many links to Sound Effects. Most appear to be in .au fomat (note: this is NOT Audio Unit), which is not compatible with GB, however you can convert them to AIFFs using iTunes with the Never-Fail™ Method
Freesound: Lots of odd little wav files you can drop in your GB timeline, or perhaps convert into AppleLoops
Break Beat SF/X: more than 3 dozen loops packages, and lots of other categories as well.