The software for editing icons needs to run on OSX Lion.
In 10.5+, Apple's built-in Preview.app lets you do edit
(Preview used to be pretty bare-bones back in 10.2 days, but with every OS release it has gotten more and more powerful.)
After opening the file, use the up/down arrow keys to switch between the different sized icon and bitmask images contained in the .icns file.
Then with each image you can:
- Annotate (draw rectangles, add text, lines, arrows, ...)
- Change transparency
- Adjust color (saturation, tint, sharpness, etc)
EDIT: This answer is now out of date. Apple recommends using "Asset Libraries" instead of creating
Apple provides Icon Composer as part of their free developer tools.
This is the only officially sanctioned way to create icns files. Most other editors don't support all the possible features.
The idea is you create a png file for each size you want, then create an icns file in Icon Composer, and import your various png files into it.
I would think that the program Opacity would be a good place to start for designing icon art as well as all manner of math and code drawing options.
It was designed from the ground up to do vector drawing and output many sizes of a master drawing as needed for icons to work well. Due to both limited resolutions for small icons as well as beefy artwork scaled up for resolution independence, the layers and tools work well.
The free trial should let you know if the tutorials and help on building icons is suitable for your skill level and needs.