Unfortunately, there is no clear answer to this in the law, and no apparent pattern to judgments. Reasonable guidelines would be to use as little as necessary, and properly cite them.
Many questions about copyright boil down to "can I do this?"is this protected?who controls it?can we use it without a license?how much can we use without a license?who do we get a license from?how long will it last, how much will it cost, and is it worth it?
With a license, yes. Without a license, no.
With a license, yes. Without a license, no.
With a license, yes. Without a license, no.
It is illegal to distill alcohol for either personal or commercial use without a federal license to do so.
A Canoe and maybe a kayak.
With a license, yes.
No; most famous characters are protected by copyright and trademark, and publishers may object to any recognizable portion being used without permission.
With a license, yes. Without a license, no.
no.
It depends on the licensing of the clip art; for example, much clip art is licensed for personal use only, so commercial use would require an additional license.
The license for CppUnit is released under a GNU Lesser General Public License, meaning it is a free software license, for use without the requirement of attributing the source code.