I very much respect someone who will use their skills and talents to develop software or a website and then let people use it free of charge. Developing something takes hours not just to write the code, but to decide on the best tools to write the website/program in. And before that, to brainstorm. And then later to troubleshoot. And finally, to get it just right! The creators have every right to charge people for what they painstakingly created, and yet they just put their stuff on the web for anyone to use.
Why?
I have a few guesses. Some are based on research, others are based on my own speculation.
(1) To Build Their Resumes
something employers really look for. Before hiring someone, a company usually wants to see what projects a prospective developer has completed or worked on so that they can get an idea of how advanced the developer is in his or her skills. Therefore, it is very common when someone is just starting out to make free programs or websites just to show prospective employers what they know and what they can do.
(2) To Improve Their Skills
For those who are with a company and don't plan to leave it anytime soon, building a resume is not as big of a priority. However, moving up in the company or being placed on new (and possibly higher paying) projects is. Therefore, these developers will generally continue to learn new languages and codes to be able to move up or over. Showing a project manager a project they have completed that works is a great way to get to where they want to be.
(3) For Fun
Linus Torvalds, creator of Linux, a free operating system/software system (in simple terms), wrote a short email back in 1991 declaring he was doing a new project “just for fun.” On the Linux Foundation's website, they write: "Today, Linux powers 98% of the world’s super computers, most of the servers powering the Internet, the majority of financial trades worldwide and tens of millions of Android mobile phones and consumer devices. In short, Linux is everywhere."
And he did this "just for fun." There is more on Linux under #5 "Because They Really are Just Good Samaritans."
(4) The Company They Work for Tells Them To
Sometimes companies want their employees to get involved in open source projects. Google, Twitter, and Netflix sponsor major open source initiatives. Why? Possibly for the tax write offs for charitable donations. Also, getting involved in open source projects encourage outsider's to collaborate on that company's code, thus making the code better. It also gives the company prestige: they get their name out there.
(5) Because They Really Are Just Good Samaritans
While there are rumors that Linus Torvalds isn't the nicest person to meet on the streets, there is no denying that he could fall under the category of a "good Samaritan." The underlying source code of Linux may be used, modified, and distributed—commercially or non-commercially—by anyone under licenses such as the GNU General Public License. What does this mean? Basically, anyone can download, use, or sell Linux and Linus Torvalds will not see a dime of it. Linux is trademarked under Linus Torvald's name, but that is it.
I have yet to find anything that says Linus Torvalds developed Linux as a public service. More of what I read tells me that he developed it for himself, for fun, and then put it on the web for others to download. Still, there is no explanation from Linus himself why he doesn't charge for an initial Linux download. He certainly could have. Microsoft and Apple did.
Other founders like Jimmy Wales (Wikipedia) and David V. Kocher and Yves Langisch (Cyberduck) fall into this category as well.
How These People Support Themselves
Sometimes free software or free websites actually do make money. Facebook is free to use, but they make money off of advertisements. The same goes for Twitter, YouTube, and Google services. Other sites, like GitHub and DearElder, are free websites to use, unless you want to use their premium site: then they charge you. Sites like Amazon, Ebay, and ODesk are free to use and buy products, but if you want to sell something, you usually have to pay them a 10-15% fee. And then there are other sites that adamantly ask for donations like Wikipedia and CyberDuck.
For Linus Torvalds, the Linux Foundation sponsors him so he can work full-time on improving Linux. The Linux Foundation is supported by corporate donations although they do not adamantly ask for donations.
Learn More
For more on this, check out these sources. Yes, some are Wikipedia, but Wikipedia is a great springboard for further research if you're interested.
Free Software vs. Open Source: Yes, there is a difference.
Linux Frequently Asked Questions
Why Corporations Are Happy To Donate to Linux