it depends on what format you have...

3dsmax has a polygon reducer that works fine.

Milkshape also has a polygon reducer which is imo a bit better that of MED (pro license only feature)

there are other free tools I have not tested yet, probably in Blender?, http://www.atangeo.com/, http://meshlab.sourceforge.net/


Free world editor for 3D Gamestudio: MapBuilder Editor