LocoSim (Popularity: )
http://www.locsim.ch/ Swiss locomotive simulation program. Free limited version for download.
Yard Duty (Popularity: )
http://www.yardduty.com/ Network based simulation of building and breaking up trains in prototypical yards. Includes screenshots and shareware client download.
3D Train Stuff (Popularity: )
http://www.3dtrainstuff.com/ Route and rolling stock addons. Some free downloads and product information. Tips and forum for 3D modelling.
Dieselswest (Popularity: )
http://www.dieselswest.com/ Add-on locomotives for Microsoft Train Simulator. GE U-Boats from 1956 to 1975. Also provides some free ware rolling stock