There are a number of people who have contributed in one way or another:
Jim Steven - many things
Raymond Kalousdian - showed how to make slips work
Jonathan Cronje - showed how to make 250M radius curves work
David Beach - initiated the standardised tsection.dat
Teemu Saukkonen - responsible for the bridges and carfloat ('7bridge' & '7carfloat').
Paul Andrews - got the subterrainian sections added
(others have suggested it before but Paul was the one getting it done)
Robert K. Guy - point animation fix for 870r & 10dY points
René Wiggerts - concrete shapes
Doug Jones - 27M radius narrow gauge curve
Teemu Vehkaoja - new 10d switches and provided input for the switch animations
Martyn Griffin - initiated the a1t30mRoundhThru
Yuri J Sos - helped with the installer introduced from version 3.7
Tim Bridge - UKN* narrow gauge track sections
Andrew Gadd - point animation fix for UKN* and new UKN* shapes.
Steven Masters - bridge rail switches and lots other bits and pieces.
William 'Bill' Adams - DualTracks package (D* and some A* & N* tracks).
Silvan Schwab - additional concrete shapes (some based on shapes by Kai Schadeberg).
Mark Hyams - smother curves.
Chris Cyko - CYKWORKS, C* & L* elevated track sections.
Norbert Rieger - additional subterrain sections, slips, switches and much more.
Zoltan Ivan - replacement shapes with improved LOD.
Timas Adamaitis - 0.3m, 1m & 5m tunnel straights both a1t & a2t.
Dereck van Dijk
and all others reporting problems and wishing for new track sections
Train Sim Modeler by Abacus
An easy to use 3D modelling program for those without prior experience, only suited for making simpler scenery objects.
gmax by Discreet
Free little brother to 3D Studio Max, capable 3D modelling program but a bit harder to learn than the above (recommended).
You will also need the Train Simulator Game Pack available from Train Simulator Insider
TGATool2 by Martin Wright
A great tool for viewing and creating ace-files (MSTS textures), can convert between ace and tga/bmp file formats (recommended).
Shape File Manager by Paul Gausden
Not a large program but very useful, allows scaling and shifting of s-files (MSTS shape files) (recommended).
SView by Paul Gausden
Also have to mention Pauls upcoming shape viewer. It's now in beta testing and it looks real interesting.
QSetup by Pantaray
Intelligent installer free for non-commercial use, supports various options and testing during setup (recommended).
Free internet domains curtesy the Polynesian nation of Tokelau.