Is there a quick way to delete all materials in the scene? I have a scene containing 20+ materials and all linked to objects. I'd like to remove all the materials from my blender file.
Yes, you can. Easy way would be:
All unused material data will be removed after saving. You cannot see them anymore after reopening.
There's an addon Material Utils, which ships with Blender.
Enable it in the User Preferences > Addons, select all objects A, hit Q over 3D View and chose Remove Material Slots:
It currently works on the active object only, despite the tooltip stating it would work for multiple objects. Here's a scripted solution for all selected, editable (non-linked) objects: