Originally Wikimedia Commons served as a central place for images, audio and video files used by different language versions of Wikipedia. An image has to be uploaded only once and it can easily be used by various Wikimedia projects. But for a long time Wikimedia Commons is used by various other external projects, in fact anyone is free to use and distribute data found on the Wikimedia Commons servers - generally there's only one condition - you must comply with license(s) the given file was published by its author. Usually it is GNU Free Documentation License or several Creative Commons licences - a part from that a lot of files are in Public Domain.
Wikimedia Commons now has over two million files. This is around 11 months after it reached one million. Since March 2007 it routinely has over 100,000 files uploaded every single month. It is becoming more and more common to have over 5,000 files in a single day. Please read more on this subject.
Some notable features of Wikimedia Commons:
- Picture of the Year competition
- Featured Images
- Quality Images
- Mayflower search engine (http://tools.wikimedia.de/~tangotango/mayflower/ )
- onsite ogg video/audio playback
- category RSS feeds - each category now has a RSS feed for new files that get added to that category. If you are interested in a topic and it has a category, you can add the RSS feed link to your feed reader
Wimedia Commons is still a relatively young project it is quite likely that quite soon it may reach the 10M files mark.