I discovered through the WordReference Forums today that the whole Grimms Deutsches Wörterbuch, which consists of 32 volumes, is available for free online. The project page for this is here; the direct link to the dictionary itself is here.
The Grimms Deutsches Wörterbuch is rather like the Oxford English Dictionary, except that it’s for German (obviously). I’m not sure as to how helpful it would be in actual language learning, but I’m sure it’d still be fun to dig around through. You can read more about the dictionary and its history here.