A standalone Windows utility for converting images to heightmaps (saved as AC3D files (.ac)) is available at:
http://bellsouthpwp.net/a/t/atomicd/ac3dplug/index.html
See the "Image to heightmap" link on the left, or go straight to:
http://bellsouthpwp.net/a/t/atomicd/...ug/img2hm.html
This utility converts images to heightmaps - handy for creating terrains, etc.
NOTE: This utility has some specific requirements (Windows OS, DirectX, .Net framework). Sad to have these requirements on such a simple utility, but those were the tools I had most readily available when I made the tool. And, as with all utilities on the site, this is basically something I wrote for myself that I've made publicly available (along with source code).
Enjoy