IconFinder is to icons what Google is to internet. There are very few resources on the web that have indexed such a vast collection of free icons for users to download.
Currently, IconFinder lets you search over 100,000 individual icons and over 175 icon sets. Simply enter a keyword to search for icons and IconFinder will display all the results. You can filter the icons by pixels and if you like something, download them as a PNG or ICO file. Clicking on the info button for any icon provides you more details about the designer of the icon, the license and the icon collection it is a part of.
If you are not sure how an icon would appear against a certain background, you can switch between white, grey and black backgrounds to test.
No sign-ups or registration required. Simply search and download.