Oh yeah, in the states, pretty much *anything* published by the Federal Government is automatically in the public domain.
There are a couple of exceptions, but that still means that there is a helluva lot of 'free' content you can legally use.
It gets really messy, when you have content that is now out of copyright in some jurisdictions, and not in others. Technically you can do as you please with it in those jurisdictions, but on the intarwebs it is open to the whole planet right (except for China sometimes).