I’ve been really into icons since I first started using computers. Customizing icons in Windows (3.1 and 95) was possible at the time, but very lame. I’m not sure it’s any different now. When I got my first Mac and found out you could just copy and paste icons between objects I was hooked. I've been collecting and studying them ever since. I even like icon design in the real world too.
From my first days on the Mac, I've turned to the Iconfactory for free icons and inspiration, and I can't recommend Panic's CandyBar enough for storing and organizing icons as well as replacing default system icons.
I've only made a few myself though, and they always fall way short of what I’d imagined. I’ll put them up here soon anyhow.
Recently I had some simple ideas for the iTunes icon, which you can see in progress here.
Here's someone else's list of nice OS X icons from 2010. Some great picks there.