Max
Wire Bundle creates a series of wires/tubes along a spline or from 1 object to another
WeldTexVerts lets you interactively weld texture vertices in the same way you would with editable polygons.
Retopo allows you to retopo directly in max.
Maya
Wire Jumbler similar to Wire Bundle max script,
X-Tools poly moddling/editing toolset
Pelting Tools tools to simplify UV-ing
Zen Tools set of multi-purpose tools for poly's, Uv's, rigging and more! (requires Bonus tools- link on same page)
Extend Duplicate duplicates objects along a curve/surface
oaRelaxVerts relaxes verts while keeping shape
P3D solidify similar to maxs shell modifier
Local Tool Orientation changes to Transform orientation to selected object, point, etc
XSI
ZBrush
Precision Texturing
Subtool Plugin plugin to manage subtools
Box Modler like Zsphere's, but boxes
ZBrush Plugins Download center Zapplink, Transpose Master and more all on one page!
ZBrush Alpha Library
ZBRush Matcap Library
Mudbox
Modo
VertexMonkey resource center for all your modo needs.
SHADERS
Max
Camouflage Textures, ie seamless
Maya
Brake Disk Anisotropic Shader scratched metal shader
denfo-Surface-of-Ice ice shader
XSI
SkinSSS skin shader
Normal Map Control allows Red and Green channels to be flipped indavidually
ZBrush
Realistic Materials small collection of, well, realistic materials
Polaris Alpha pack set of 83 alpha maps from Polaris
Mudbox
Modo
X-TRA'S
Slipgatecentral's DWIV video tutorials
X Normal
Marmoset Toolbag
Polycount Marmoset thread for all things releated to the toolbag
Zbrush and Mudbox Stamp thread our very own stamp thread (this was a hard find).
Mudbox - use geometry as a brush video tutorial on using complex geometry as brushes
Luminxo Puupetshop for Max
GAME ENGINES and TUTORIALS
General multiple engines/tuts/etc
Devmaster
fpsbanana
ModDb
the following is stolen directly from polycount
Texturing Tuts by Daniel Vijoi (Lead Texture Artist of AMC Studio) Download a PDF from the share section at AMC Studio
Creating a Next-Gen Environment Textures
Creating a Next-Gen Enrionemtn Setting- Creating an Old Farm
Hard Surface Texture Painting by Stefan Morrell
Modeling for Next-Gen Game (Useful notes on normal mapping)by João Campos Costa
Creating a Current-Gen Dumpster by Kaleb Aylsworth
Also Gears2 Environment Art thread by Kevin Johnstone beggars belief as well as having a some step by step's a few pages in.
Unreal
waylon-art.com
UDN
Epic Games Forums
Source
Valve Developer Wiki
Interlopers.net
EditLife.net
Steam Tools Forum
CryEngine 2
Crymod