Well, Nordschleife is the obvious one offcourse..
The Dutch circuit Zandvoort had it's last trackday friday the 4th, due to a lack of interest.
In Belgium, Circuit Zolder has regular trackdays, check
Circuit Zolder for info on the circuit, and
Eran Racing for trackday info.
Spa-Francorchamps does have some trackdays, but not very often..
The UK is trackday capital of the world I guess, a lot of organisers and tracks overthere.. ie
UK Trackdays
Hope this helps..
Gerrit