Other than Tango Top Quality Vector Icons, there is another set of professional high quality icons – GNOME 2.18. GNOME 2.18 Icons are in PNG ( Transparent background which can easily integrate into any web site) and SVG format ( Scalable which means you can increase the size of the icons without losing quality. There are over 600 icons altogether. I think this is one of the best icon sets I have ever seen.

Requirements: No Requirements
Demo: http://art.gnome.org/themes/icon/1352
License: GPL License