Apple’s Logic comes with plenty of nice loops. These files are stored in “/Library/Audio/Apple Loops/Apple/Apple Loops for GarageBand”. However the files are in .CAF file format (Core Audio File). OS X comes with tool to convert these files, called “afconvert”.
To convert the files to for example AIFF (which is supported by Ableton Live) you can use command:
afconvert -f AIFF -d BEI16 infile outfile
You can find a script to batch convert all files from here: https://gist.github.com/1073217
MainStage 2 comes with plenty of nice samples (with the Jam Pack extensions which can be freely downloaded within the application). These files are stored in “/Library/Application Support/GarageBand/Instrument Library/Sampler/Sampler Files”.