Please post links to any free resources you know of and if possible credit the creators. I (and any moderator that can spare some time) will add them to the master list in this very post.
Special thanks to all the resource creators and the people who have submitted the links.

There have been some great submissions since I last updated this master post, this link will take you to the last one I added, so it's worth foraging for anything past the link.

Free Software:

Game Engines - The one & only!!!

Texturing & Image Tools: - Free substances!! - Generate textures in browser - Free texturing tool from Allegorithmic (makers of substance) Dead link - Generate normals in browser - Image Editor/Painting program - Image Editor/Painting program
Irfanview - Open, convert, optimize, and use batch processing (renaming for example) for nearly any image type.
SpaceScape - Create an outer space skybox with stars & nebula's.
xNormal - Bakes out normal/AO/height and many other maps from a high poly mesh.
handplane - Tangent space calculator for converting Object space normal maps.
RoadKill UV Tool - Unwraps your models from 3dsMax/Maya/Softimage.
Wood Workshop - Wood texture generator (credit to 94)

Modelling, Sculpting, Animation and Mesh Optimization Tools: - Free sculpting + painting tool from the makers of ZBrush - Free modelling/rigging/scuplting/animating tool - LOD generator and mesh optimization (cloud based, free for personal use)
MakeHuman - Free 3D Humanoid character creator (Open Source).
3dsMax Rock Generator - MaxScript for quickly creating high quality rock meshes.
Autodesk FBX Converter - Quickly convert OBJ/DXF/DAE/3DS files to an FBX format file.
TerreSculptor - Currently in beta, is very similar to World Machine. - modelling app - Foliage generator - Terrain generator
IKinema WebAnimate - Free web based animation for your 3d characters. No longer free?? - Free Apprentice edition of Houdini (watermarked and of limited use but good for learning) - 'Fragmotion can be used without buying it (for 50 USD), if you type in the prayer text appearing weekly' - A mesh mashup tool from Autodesk, some pretty cool sculpting tools too - I just found this yesterday, now my all time favorite 3D modelling program - it's the only one I am capable of making anything with :/ - Auto rig meshes (with source code), would be good if this could be turned into a simple UE4 plug-in (hint hint)

Audio and Sound Tools: - Audacity, a free audio editor - an awesome online sound effect generator - Free DAW - VST compatible or standalone synthesizer - Free open source DAW

Other - Open source launcher - Compositing package - Live streaming and recording - Community edition of Visual Studio (highly recommended!!) - Capture gif's - Tiled 2d map editor

Free Assets/Resources:


Animations & Motion captures:

Models, Materials & Textures: - fighter5347's incredible gift - Thanks to DaveFace!!

Materials and Textures:

Textures only:

Blueprints/UMG/BT & Project templates: - Dynamic Crosshair (thanks to Osman) - Networked UMG menu (thanks to SaxonRah) - Inventory (thanks to ruohki) - Weather & Ocean (thanks to DotCam + friends) - AI templates (thanks to Peter L. Newton) - SaxonRah's UMG online subsystem - Courtesy of Mhousse1247 - Courtesy of Mhousse1247 - procedural physics enabled rope bridge

2D Sprite Sheets & Background Textures:

99Sounds - A ton of high quality sound fx collections.

Tutorials/Books - Some great tutorials and example projects to get you started in UE4

IES Lighting Profile Archive - A great resource for IES photometric lighting textures (Lithonia Lighting).

*Disclaimer - I have not had the time to test all of the links, assets and software, therefore I will not be held accountable for any damage these may cause to your computer, zen, ch'i, karma or well being