Is there a SVG symbol library for QGIS?

You'll find some symbols and the way to install them on http://ssrebelious.wordpress.com/2011/10/11/qgis-symbology-set/:

1. Use QGIS 1.7 or higher.
2. Go to Settings->Style manager.
3. Hit “Import” button.
4. Provide path to the “symbology-ng-style-eng.xml” file in this archive.
5. Pick up needed symbols or choose all of them and hit “Import” button.

Not QGIS-specific, but Maki is a map-specific icon set in SVG from MapBox.

Tho it's not a set of Symbols, in the noun project you can download many SVG files (under creative commons licence) that can easily used to create composed symbols in QGIS.