ARMIJN HEMEL, 32, lives with his parents in Tiel, a town smack-dab in the middle of the Netherlands. He works as a technology consultant, but spends several hours a week on his avocation: pestering some of the world’s most powerful consumer electronics and technology companies.

Mr. Hemel serves as a volunteer watchman for free, open-source software like the Linux operating system, which competes with Microsoft’s Windows. The use of free software has exploded, particularly in gadgets as varied as exercise bikes, energy meters and smartphones. Companies like Google, TiVo and Sony often opt to piggyback on the work of others rather than going through the ordeal of building all of the software for their products from scratch.

The problem that Mr. Hemel and others have stumbled upon is that some companies, even some technology savvy ones, may be violating the rather easy-to-follow requirements associated with free software licenses. Typically, these include making tweaked versions of a free software product available to the public, or simply giving credit to the original developers.

Last month, for example, Dell received a public tongue-lashing from the geek kingdom and a cease-and-desist letter courtesy of Mr. Hemel for shipping its new Streak tablet without providing the underlying open-source software code. Dell representatives acknowledged the issue and later put the code on a Web site. “We are committed to fulfilling all of our obligations when using open-source code in our product,” said a Dell spokesman in a company blog.