OpenIcons is a set of SVG icons licensed under the LGPLv3 that are intended to be used on the web. All the icons are vector based and made using InkScape. The icons can easily be converted from SVG to pretty much any common graphics format.
Since the icons are meant to be used primarily on the web and in mobile applications the default distribution of the icons includes PNG copies of all icons scaled to the following sizes 16x16, 24x24, 32x32, 36x36, 64x64, 72x72, 96x96 and 128x128 pixels.
The reason why I started this project is because there is a shortage of icons suitable usage for the web licensed under a GPL, LGPL, Apache or BSD compatible license. Most artists seem to choose to license their work under the Creative Commons family of licenses which is not compatible with GPL or LGPL license and therefore causes some significant problems if you want to have a codebase that's 100% GPL or LGPL compatible.
For more information on the background of OpenIcons see this blog post.
Here is a few samples of an icon scaled to 16x16, 32x32, 64x64 and 128x128 pixels.
New icons are added to OpenIcons continually as the need for new icons emerges and as contributions come in. Here is and index of all currently available icons.