Apple - Is there a decent icon editor for OS X capable of editing .icns files?
In 10.5+, Apple's built-in Preview.app lets you do edit .icns files!
(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:
- Cut/Copy/Paste
- Annotate (draw rectangles, add text, lines, arrows, ...)
- Change transparency
- Scale
- Adjust color (saturation, tint, sharpness, etc)
EDIT: This answer is now out of date. Apple recommends using "Asset Libraries" instead of creating icns files.
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.
Is depends on the kind of editing you need to do. icns Editor is on the App Store. Maybe this one too?