Hi aaronnordstrom, thanks for the question! Perhaps other Revit users can also chime in with some personal favorites, but in the meantime, let me point you to this post on our blog, which covers several free resources for high quality textures. Another great site that isn't included on the list, but which I would definitely recommend is Texture Haven. If you'd like to go a bit deeper and create materials yourself, you might also take a look at something like Substance Designer.
I hope this gives you a jumping off point for your material library! 
Thanks, Helen!
This is very helpful. When it comes to pre-made revit materials (with settings tweaked), there are no resources as far as you know? In the future, some sort of clean and accurate Revit material library with a few dozen textures would be an awesome package that Enscape could sell to help us get started.
No resources regarding pre-made Revit materials I know off, but I definitely recommend Texture Haven, which has a lot of great free textures! With a bit of tweaking in Revit itself you'll surely achieve nice results.
Also, your further feedback will of course be forwarded - thanks a lot for that as well.
My blog post may help you with making your own textures: http://revthat.com/how-to-make…ing-texture-from-a-photo/
+1 for a material asset library for revit. Common things like grass, roads, concrete and pathways to quick start some materials would be good.
Thanks Damo , +1 for the corresponding topic on our agenda, aaronnordstrom , thanks for the further feedback. Will be added to our agenda as well! 