The AGE system has been mentioned more than once but really encompasses a lot of properties you can play in:
- Generic Fantasy AGE (which is getting a new, updated core rules manual), can be played in Freeport, Blue Rose, Lost Citadel, and Titansgrave.
- Dragon AGE (the progenitor of the entire AGE system) I ran a Dragon AGE campaign for years and it was a blast. There was even a Midgard bestiary produced by Kobold Press at one point. Ran a converted N1 Against the Cult of the Reptile God in this system, it was awesome.