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)