The web hosting portal FatCow distibutes an amazing set of over 3900 icons for free: the icons are available at this link.
These are, however, quite challenging to browse locally under Windows as the OS takes a while to render them. Someone smarter than me previously published a simple script to browse them online, under this repository.
Long story short, Hunter Wu's html refers to an older iteration of the FatCow collection, I simply extended their script adding the missing icon and support for both 32px and 16px icons.
Head to the live version to search and download the icons, or clone/download the repo to use it locally.
This repo is unlicensed.
Be aware that the FatCow icons are licensed under a Creative Commons Attribution 3.0 License: check their terms of use for further info.