Public domain content in practices any content without copyright. It may be deliberately freed from restrictions by the copyright owner, or the copyright may lapse after a certain time. All content created by US government employees as part of their employment is public domain, making US federal sites (considered together) probably the largest source of public domain content by far.
The legal details in practice vary between countries. More information about public domain can be found at Wikipedia's article on the public domain.
Creative Commons tagging of Public Domain content
Creative Commons' public domain dedication is now available.
CCZero is a new mark which improves and extends this.
Appropedia's Public Domain Search
Appropedia's Public Domain Search was started in late 2007 when it was discovered there was no effective public domain search available. This operates on a completely different method to searches using the Creative Commons marks. It uses a manually maintained index of sites known to be public domain - thus it is does not yield 100% public domain results, and content must be checked to confirm public domain status. Reliability is expected to improve, and feedback by users is encouraged.
As the CC public domain declaration and CCZero mark become widely used, searches based on the CC marks would be expected to take over from Appropedia's Public Domain Search.