UIU is a suite of several applications for 3d artists and game developers. Primarily focused on the creation of environments and environmental effects, it's a good tool that's easy to learn. All components come with a help file and quick start guide to get you up and running quickly. It includes the following tools:

Eleblend - Texture blending for terrain
Ripple Rain - Alpha channel rain and water impact ripple animation
Gradient Thief - Multicolor gradients from existing images
UT Screenshot Assistant - Quick image processing for Unreal Tournament 2003
Mosaica - From image to mosaics with matching bump maps

More about it and download here!