Frico is a free icon maker app that can load jpg, bmp, png and gif files and create an icon file that includes smooth transparency for both Windows and Mac OS X in sizes ranging from 16x16 to 256x256 (for Windows) and 16x16 to 512x512 (for Mac). You can pick specific graphic files for a specific size, or load one general file for all sizes.

The program includes a preview of all the sizes your icon can be displayed at so that you can see the end result without fiddling with folder settings after you're done.