Bitmap to Icon Wizard lets you create high quality icons of all sizes from bitmap files. The program has support to generate icons with transparency effects, even those of 32-bits, as long as the bitmap has a well-defined background colour.
Bitmap to Icon Wizard has support for up to four colour depths (32-bit with alfa channel, 24-bit, 8-bit, and 4-bit) and five possible icon sizes: 256x256, 48x48, 32x32, 24x24, and 16x16. The application applies octets to create images with 256 colours, via the Floyd-Steinberg method. Of course, you can always choose the colour depth and size you use for each icon you create.