Wikimedia Commons is a media file repository making available public domain and freely licensed educational media content (images, sound and video clips) to everyone, in their own language. It acts as a common repository for the various projects of the Wikimedia Foundation, but you do not need to belong to one of those projects to use media hosted there.

Unlike traditional media repositories, Wikimedia Commons is free. Everyone is allowed to copy, use and modify any files here freely as long as they follow the terms specified by the author; this often means crediting the source and author(s) appropriately and releasing copies/improvements under the same freedom to others.

[From the official website]