Q: What resources(products/works created by others) in the public domain can be used freely for commercial purposes?

A: If something is truly in the public domain, then it can be used by anyone for any purpose. The trick is being able to tell for sure whether the work you want to copy is truly in the public domain.

You can find a definition of the types of "works" that are protected by copyright in Circular 1 at copyright.gov. Circular 33 describes things that cannot be protected by copyright, and so those items of intellectual property would be in the public domain unless they are protected by trademark or patent law.

If you know that a work was created in 1924 or earlier, then that work is now in the public domain. Works created after that date may or may not be in the public domain, depending upon whether they were properly registered or renewed under the law that was current at the time.

Works created by the US Government (employees) are all in the public domain.

An author can declare that some or all of their work is in the public domain, and allow anyone to use it as such. You should be careful about relying on statements like this, in case the statement turns out to be a forgery, or the person dedicating the work to the public domain isn't really the person who has the rights to do that.

A: Patents that are expired are also in the public domain and can be used. But you have to make sure that they are indeed expired and no continuation or divisional patents or pending applications have issued out of the expired patent.