Prior art
Everything publicly known before a patent application was filed – and the sharpest weapon against a patent that should never have been granted.
An invention has to be new to be patented, and novelty is judged against prior art: earlier patents, papers, manuals, products, conference slides, ancient Usenet posts. Examiners search, but they can't search everywhere, and software is harder than most fields because much of the relevant art was never published in any database – it just lived inside a program.
That makes prior art a strategy as well as a test. Publishing an idea openly, a so-called defensive publication, stops anyone else from patenting it later. Entire campaigns against patent trolls have consisted of digging up an old manual or mailing-list thread proving the "invention" existed long before the patent did.