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. You can see the entire set in the screenshots below:
The set is comprise of about 1,000 icons which cover a wide range of application needs: documents, folders, users, calendars, disks, databases, feeds and so on.
The icons are 16 pixels by 16 pixels and as Mark puts it, "are in strokably-soft PNG format".
The icons are licensed under a Creative Commons Attribution 2.5 License. They can be used for any purpose and you're free to make any modifications. The only requirement is that you include a link back to the "Silk" icons page on the FamFamFam.com site in your credits.
This site makes use of some icons from the "Silk" set. The bullets in the lists of this blog's sidebar are slightly-shrunken versions of "Silk" icons.
