Out of the box, I can see the above plus: douban, edelweiss, overdrive, ozon.
The Amazon id also has different country site versions as well.
There are also some metadata source plugins that add them as well. I can see I have ff (Fantastic Fiction), goodreads, fictiondb and barnsandnoble.
And as itmpi said, you can create arbitrary ids. They will show in the tag browser, but not in the details pain.
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