Urho3D is greatly inspired by OGRE (http://www.ogre3d.org/) and Horde3D (http://www.horde3d.org/). Additional inspiration & research used:

Rectangle packing by Jukka Jylänki (clb) (http://clb.demon.fi/projects/rectangle-bin-packing/)
Tangent generation from Terathon (http://www.terathon.com/code/tangent.html)
Fast, Minimum Storage Ray/Triangle Intersection by Möller & Trumbore (http://www.graphics.cornell.edu/pubs/1997/MT97.pdf)
Linear-Speed Vertex Cache Optimisation by Tom Forsyth (http://home.comcast.net/~tom_forsyth/papers/fast_vert_cache_opt.html)
Software rasterization of triangles based on Chris Hecker's Perspective Texture Mapping series in the Game Developer magazine (http://chrishecker.com/Miscellaneous_Technical_Articles)
Networked Physics by Glenn Fiedler (http://gafferongames.com/game-physics/networked-physics/)
Euler Angle Formulas by David Eberly (https://www.geometrictools.com/Documentation/EulerAngles.pdf)
Red Black Trees by Julienne Walker (http://eternallyconfuzzled.com/tuts/datastructures/jsw_tut_rbtree.aspx)
Comparison of several sorting algorithms by Juha Nieminen (http://warp.povusers.org/SortComparison/)

Urho3D uses the third-party libraries list of which may be found in README.md

DXT / ETC1 / PVRTC decompression code based on the Squish library and the Oolong Engine.
Jack and mushroom models from the realXtend project. (https://www.realxtend.org)
Ninja model and terrain, water, smoke, flare and status bar textures from OGRE.
BlueHighway font from Larabie Fonts.
Anonymous Pro font by Mark Simonson.
NinjaSnowWar sounds by Veli-Pekka Tätilä.
PBR textures from Substance Share. (https://share.allegorithmic.com)
IBL textures from HDRLab's sIBL Archive.
Dieselpunk Moto model by allexandr007.
Mutant & Kachujin models from Mixamo.
License / copyright information included with the assets as necessary. All other assets (including shaders) by Urho3D authors and licensed similarly as the engine itself.