Icon Finder: an awesome search engine for icons
Heads up! This post was written in 2009, so it may contain information that is no longer accurate. I keep posts like this around for historical purposes and to prevent link rot, so please keep this in mind as you're reading.
I came across a really awesome icon resource awhile back, but never got a chance to write anything about it. If you're a web designer or developer who always has use for quality icons — especially ones with GPL or similar licenses — this is the website for you. It's essentially a search engine for icons. Everything about this website rocks, especially their logo.
Not only can you filter by license type and image size, you can also download both the PNG and ICO versions on a black, white, or transparent background. You can edit the tags for any icon and, if after all that you still can't find the icon that's right for you, you can opt to purchase a custom icon design.
The website doesn't have a massive selection of icons to choose from yet, but you should find something relevant to what you're looking for. If not, you can always design it and submit it yourself.