The Spore Creature Creator is a preview and demo for the full Spore game, which is scheduled for release on September 7. As you might expect from the name the Creature Creator allows you to build and customize creatures which you'll later be able to use in gameplay once the full game is released. I've been playing with the trial this morning and it does look very promising.
The Spore Creature Creator is Leopard and Intel Mac only. It requires at least an ATI X1600 or NVidia 7300 GT with 128 MB of Video RAM, or Intel Integrated GMA X3100. It will apparently not run on computers with the GMA 950 integrated graphics chipset on OS X (though, it will on Windows). The trial version can be downloaded directly from the Spore trial site.