Awhile ago I discovered something that maybe only a nerdy designer like myself would care to discover. Via one of my favorite designers, Jessica Hische, I found that one can change the image for application icons (Safari, Quicktime, iphoto, itunes, etc.) which would create a fully custom & synchronized look to the dock on my mac. She has a wonderful blog post about it, check it out here for more.
For someone who may be a bit of an aesthetic-control freak, this was a glorious revelation. The original icon images for programs appear somewhat ordinary after awhile, causing your iMac dock to become un-inspiring. What better way to liven up your creative workspace than to custom design those icons!
Now once you have an .icns file, you can make it simple and use a program called Candybar, which will basically do all the technical grunt work of changing your icons for you. It makes it super easy, but the software costs $$ so that might be a deterrent for some...
If you'd rather do it manually, I really couldn't explain it any better than Jessica already has with this short & sweet instructional video. See it at the end of her blog post that I mentioned before. Thanks!