[falcolas]

That's actually incorrect in the US. The default state of a work of art is that it is copyrighted with ownership granted to the creator, with all rights reserved.

These rights have to be waived for it to become public domain.

http://www.copyright.gov/title17/92chap1.html#102

[pyre]

It's actually difficult to explicitly put your work in the public domain. There are no well-worn routes to doing so.

[russellsprouts]

The Creative Commons level zero license is probably the best option. In some jurisdictions it is impossible to give up something into the public domain, so the CC0 explicitly gives up as many rights as possible. If it is possible to put something into the public domain, then it does.