Platinum Arts Sandbox is a game design tool that allows users to quickly and easily create their own worlds, even cooperatively.  It is free, open source, and easy to use.  In addition it has even been used, and very well received, at Mildred Elley College during the summer Kids Camp in a course on how to create modifications.

The main feature in this release is the addition of many new models. I included a map called modelshowcase that you can access from the menu which has all of the models in game lined up according to how they appear in the menu list.  Models can be dropped into the map by entering edit mode and then hitting ESC to enter the menu. Once in the menu select editing/ents/mapmodels and select the desired model. It helps if you click the spot on the ground you want the model to appear at. For fast duplications of models select a model, hit C and then select where you want to copy it to and hit V. Also to see the models properly make sure you are in edit mode and hit L for fullbright mode.

This release is mainly thanks to these great guys who graciously let me use their media for this release:
aard: Cube 2 Engine and graphics
DCP: Models
Geartrooper: Models
KaiserTodretter: Models
Kurtis84: Models and Graphics
Makke: Graphics and Models
Nieb: Models
Tentus: Models

You can download the files here:
http://www.kids.platinumarts.net/fireboard/Downloads-Tech-Support-and-Feedback/