Public Domain Search

One of the big advantages of open content is that there are no problems with indexing it - unlike proprietary stuff, where owners can get unreasonably jumpy at the idea. Public domain materials are the ultimate in openness, and here's a basic search engine for some of them:

major public domain sites were chosen, the most important being the US federal government sites. government:

* .gutenberg.org
* .fed.us
* .gov
* .mil

But there are plenty of exclusions. Also, it's a pity this is only for the US: the public domain is somewhat bigger. (Via Open Access News.)