This has been a favorite for some time. The format has changed with its change in hosting, but it has been referenced in a number of threads.
Formerly, its host site was www.acronet.net/~robokopp etc. which was linked in many threads. A primary source for German songs, but it pretty well covers the waterfront. Richard Kopp is most knowledgeable.
Another source for German Volkslieder AND some 20,000 folk songs in all: Ingeb
Frank Petersohn of British Columbia is the master of this wonderful source.

Another source for German Volkslieder und Hymnen, with text and melodies, is Index A

And don't miss Volkslieder Midi Collection: Midi Collection
or-

Markomannia, with its "virtuelles Liederbuch" has good midis and texts: Markomannia (Click on "virtuelles Liederbuch" left side of page)