There's the TESA resource kit, and Insanity has his own thread with a huge number of resources. You might also want to check Celtic Decor, the stuff Blary made, and the resource pack by Oaristys. That list of five covers 90%+ of the resources that are actually worth using, unless you plan on going for scripts. (In which case, there are a few others worth mentioning, but the list remains pretty small.)