"I was involved in some patent litigation [at Sendmail] and we were not aware of this option, and I wish we had been, because we ended up spending a ton of money. Everything turned out okay in the end, but it would have been nice to know about this," said Olsen, who is now a partner at Olliance Group, which provides consulting services to open-source companies.

"The important thing is that this strategy costs a tenth as much as traditional patent litigation. One of the main appeals of open source is that it's less expensive, but that also means open-source companies have less money to play patent games with," he said.

The process involves filing a request with the USPTO, which decides whether significant questions have been raised about the patent's validity. If it considers they have, the request goes to an internal board for review. Crafting an argument for overturning a patent requires a lengthy dissection of it, Lindberg said. Patent holders themselves sometimes ask the USPTO to look at their own patents for assurance they are valid, a process sometimes called "whitewashing." Those requests demand far less investigation, Lindberg noted.

There can be downsides to an open-source company filing for re-examination, Lindberg noted. If the request is denied, for example, the patent is effectively strengthened because it has been "twice blessed" by the patent office.