Where to get assets for your next top game
Several different asset kinds for free (2D, 3D, Concept Art, Texture, Music, Sound Effect and Document).
Guide by Tom Looman about Content Naming Convention, good read before starting to gather assets.
Marketplace for assets, not specific for Unreal Engine, but can be used in most cases
Models with very diverse quality, but many for free commerical use
Blend Swap is a community of passionate Blender Artists who share their work under Creative Commons Licenses
A model search engine, finding models in several forums and other asset stores
Free models of the never released game Grindstone. Modern weapons and modern room interior
Thousands of free models of different categories