At Cameron Moll's site, Authentic Boredom, he's posted all sorts of media from his presentation, Essential Web Skills, which he made at the Webmaster Jam Session in Dallas, Texas. Among his slides is this gem, which explains 9 skills that separate the good designers from the great ones:

Even if you've got a full-time graphic designer at your disposal, going with standardized icons is a good idea. They'll give your designer more time to work on the graphic requirements that are unique to your application. Furthermore, we've established certain conventions that define the "language" of graphical user interfaces over time, so in most cases, it's probably a bad idea to re-invent icons for common elements like documents, folders, users and so on.

Luckily, there are a number of good icon sets for sale, but I'm going to talk about the free ones today. The MaxPower.ca blog maintains a list of sites that offer icon sets that are both "free as in beer" and "free as in speech" (they're public domain or licensed under one of: Creative Commons, GPL or LGPL).

My favourite of these is the "Silk" set by Mark James, and you can get them on this page at his site, FamFamFam.com. I've got screenshots of the entire set after the jump.