A comprehensive reference database of dimensioned drawings documenting the standard measurements and sizes of the everyday objects and spaces that make up our world.

You can select an icon (or upload it) and then fully customize it via this generator.

"The goal of this project is to offer a consolidated library of icons used in various libraries, window makers, themes and other applications. Over 10,000 unique icons in svg, png, xpm, ico, and icns formats. See homepage for details."

Create a favicon out of existing images, which'll get converted and downscaled to a favicon size. Reasonably good downscale.