Simple, Pixel-Perfect & Vector Icons: iconmonstr
iconmonstr is a free set of mono-colored and very simplistic yet beautiful icons.
The set includes 600+ items and it is possible to find multiple variations for almost each element.
Icons are mostly focused for web apps as most of them are actions, charts, badges or social networking-related.
They are available as PNG + SVG files and completely free to be used in free + commercial projects.
P.S. The only bad part is that the set is not offered as a single download pack but you have to browse pages to view/download each of them.