Owning three of these sims, each with my own preference of stock in them I thought it might be nice to share some of these links with you:
MSTS
www.uktrainsim.com (a huge library of rolling stock and locos - mostly from britain but with a few foreign locos if you look) and basically anything else you want for MSTS!!!!
http://www.eutrainsim.com (a library devoted to European downloads)
http://steam4me.railpage.org.au/trainsim/ (another good site with mainly austrailian locos,a few useful facilities here (Route Riter, Shape Viewer etc.)
http://www.train-sim.com/ (another huge library of basically anything you want - more of a US theme though this time!)
http://www.bubblesrailways.co.nr (my own site with a few locos and soon my RH&DR stock.
http://www.freewebs.com/mvts/ (a site with a few downloads and several other things!!)
http://www.highworth.freeuk.com/ (The Swindon and Highworth Railway for MSTS - Route updates and rolling stock!)
http://www.bangor.ac.uk/ml/whr/msts.htm (downloads themed on the Welsh Highland Railway in Wales)
http://www.spoonersts.co.uk/index.html (the people creating the Welsh Highland Railway in MSTS
http://www.festrail.co.uk/downloads.htm (downloads themed on the Ffestiniog Railway in Wales
http://www.talyllyn.co.uk/downloads/index.html (downloads themed on the Talyllyn Railway in Wales
http://www.freewebs.com/woottonts/ ( Another website devoted to creating a high detail fictional route for MSTS)
Trainz
http://www.festrail.co.uk/downloads_for_trainz.htm (Downloads based on the Ffestiniog Railway in Wales
http://www.auran.com/TRS2004/DLS.php The official auran download library
http://www.btinternet.com/~michael.banfield/trainz/index.html (a website with a few diesel and track downloads
BVE
http://bve-routes.com/index.php?lang=bri&country=bri (BVE Library of Routes and Links)
http://mackoy.cool.ne.jp/ (Get BVE from here)
http://www.trainsimcentral.co.uk/ (downloads of routes and locos available from here)
http://www.trainsimcentral.co.uk/ (another site with downloads of routes and locos available)
http://joshimsbveroutes.gamehorizons.net/bus/index.html ( a smaller site with downloads of the DLR and WIP shots of the Oxford Bus Route and the Metropolitan Line)
This is not an exaustive list but it is just a few of the sites that I visit a lot - please feel free to add more links.
Ben