Although I'm not much of a programmer, I'm a big fan of Open Source Software. Freeing source code for tinkering by random interested parties has produced a ton of awesome things I use all the time (Linux! Gimp! Libre Office! Inkscape! Audacity! Freakin' VLC, peoples!)
But there's a weird misconception about open source software (OSS) licenses that I've run into a number of times - apparently, quite a few people think that OSS licenses prohibit later users from commercially exploiting OSS code.
If my experience is anything to go by, the programmers, computer scientists, OSS advocates and other licensing junkies among you are thinking, "Wow, that's a ridiculous misunderstanding!" And many of the rest of you are thinking, "Well.. they do, don't they? How could anyone commercially exploit something that's given away for free?"
It seems to be a pretty fundamental cultural mis-communication. So if you're in the former group, consider this post just a heads-up, info-sharing, "hey, you might want to check if the people with whom you're communicating are clear on this point!" If you're in the latter group, here's a quick clarification:
There are a variety of free/libre/open source software (FLOSS) licenses. Pretty much all of the OSS licenses allow re-use of open-licensed code by third parties, but most also require that third party users must themselves make their code freely available. Apparently, many folks understand the requirement to make code available as precluding commercial use by third parties. But this is not true!
There are quite a few ways one can commercially exploit OSS code while also fully complying with the "open" requirements. One good example would be the Red Hat company. While the code of most Red Hat products is itself openly accessible and free for re-use, people give money to Red Hat for the convenience of getting a well-curated set of open source software, for the reliability ensured by the brand name, for product support, for enterprise-level solutions, and for many other reasons.
One factor that may contribute to this confusion is that Creative Commons licenses, the most common open content licenses (i.e., for freely sharing text, images, audio, video - rather than software code), have an option for licensors to indicate that they are only licensing "noncommercial" use. With the correct combination of terms, a Creative Commons license may enable free re-use for anyone except commercial users. Although there are some major differences between open source software licenses, the major OSS licenses enable free re-use for anyone including commercial users.
I'm not deeply versed in the arcana of FLOSS (or even all of the arcana of free/libre vs OSS), so if I got something wrong, please correct me in comments.
FWIW, my favorite public license remains the WTFPL. (Depending on your work environment, possibly NSFW.)