Simutrans

GameHippo.com (9 out of 10) (90%)

The game has graphics good enough to get the job done, and simple enough to allow it to run on slower machines. The controls are mouse-driven, which makes for a very simple scheme to learn. If you are in to sim-games like Transport Tycoon or SimCity, then this will be an excellent download for you.

Curly's World Of Freeware (4 out of 6) (67%)

This I have found to be somewhat similar to Transport Tycoon, but it's free! A little hard to learn, but there is a manual on the site.