Free

BugWorks
2D robot simulator for Java in which you complete missions by changing the behavior of bugs.
http://www.cogs.sussex.ac.uk/users/christ/bugworks/

BVE World
Detailed information and links about train routes and subway lines written from around the World for BVE.
http://www.freewebs.com/bveworld/en/

Civil
Take part in scenarios set during the American Civil war. (Open Source)
http://civil.sourceforge.net/

Critical Care
Accumulate wealth at the expense of others' suffering by picking up patients with your ambulance in this 3D driving fantasy for Windows. [Open Source, GPL]
http://critical-care.sourceforge.net/

Dismount 4T
Fansite dedicated to entire Dismount Series. Features information, screen shots, help, forum, tips, updates, scores, and downloads.
http://www.dismount.4t.com/

Fly8
Open source flight simulator for UNIX and Windows.
http://samba.org/ftp/eyal/fly8/

Free Reign Main Site
3D city simulator for Linux.
http://freereign.sourceforge.net/index.shtml

gLife
Artificial life simulation for Linux that takes into account terrain, age, sex, culture, and movement.
http://glife.sourceforge.net/

Horrum
3D future racing game for Windows with infinite road. You must set the best time between two checkpoints.
http://mordred.8m.com/horrum/

KHR and JTK's World of Stunts Driving
An old 3D car driving game for DOS in which you enter competitions with monthly changing tracks.
http://www.kalpen.de/

Magic Reel
Play this slot machine for Windows.
http://www.softlab.ntua.gr/~tatsis/

MyReels
Slot machine styled game for Windows with multiple pay lines.
http://members.ozemail.com.au/~lmstearn/

Simulator Rival
Advocates offbeat simulations. Provides information and downloads.
http://www.geocities.com/simulatorrival/

Speed Delight
3D car racing game for Windows running in single player time attack mode.
http://mordred.8m.com/speeddelight/

Sticker Book
Lets you place images on a background scene. You can resize, recolor and reorient the stickers that you use. (Linux)
http://users.powernet.co.uk/kienzle/stickers/