Icon Finder is a comprehenive database of icons that you may use in your website, mobile apps, presentation and other design projects.
You can browse icon by tags or by icon sets. The search results can be filtered by icon sizes (resolution) or by the background color of the icon.
The icons available on Icon Finder can be downloaded in either PNG or ICO format but make sure you read the licensing terms as not all icon images may be available for commercial use.
IconArchive.com and IconsPedia.com are the other popular sources for downloading icons.