This tool for viewing Gerber files in 3D is simple and free.
Jérôme Vuarand has been hard at work creating a fun, free, and simple Gerber visualization tool, GRBV. Simply drag and drop your Gerber file on to it, and you get a nice 3D rendering of your PCB.
Available under an MIT-style license, GRBV runs only on Vista or newer versions of Windows. If you haven't jumped past XP yet, you're out of luck. If you have made that jump, you can download the tool for free and play with it. It comes with some demo PCBs you can look at if you don't have any of your own to try out.
Programmed in Lua, this tool lets you dig in and modify the program without recompiling if you'd like to do a little customization. Vuarand also offers a few command-line examples of ways to customize your experience.
The program is open-source, so feel free to explore the code. Vuarand is hoping to support it as well as he can in his spare time. If you find any pressing bugs, you can email him directly. He does have other obligations, though, so he makes no promises about feature requests.