This page is about free software which runs on Microsoft's Windows operating systems. The meaning of "free software" here is software whose users are permitted to use it for any purpose, to study it's internals (source code), to modify it's internals, and to redistribute modified or unmodified copies. For the complete definition of free software, see The Free Software Definition.
For a more comprehensive list, see gnu.org's "Free Software alternatives to proprietary applications on the Microsoft Windows OS" or for a list of free software that works on MS Windows, MacOS, and GNU+Linux, or for which there are equivalents for each of those platforms, see Graziano Sorbaioli's software page (previously at chi3.org).
Below I have just listed some popular and particularly easy to use examples.
I haven't used any of these, but others have said they're good.
The value of free software is easy to misunderstand. The four freedoms are of questionable value individually; their significance arises when they are combined.
The first freedom: "The freedom to run the program, for any purpose". Some software licences prohibit the running of a program for the purpose of studying the programs behaviours. Such clauses are aimed at hiding spyware, or at inhibiting the development of similar software — which might affect the profits of the proprietary software.
The second freedom: "The freedom to study how the program works, and adapt it to your needs". This means that software users are not under the control of the software owner (the publisher). Many people discount the value of this freedom because they say "I'm not a programmer, this freedom is useless to me", or programmers say "I don't have time to reprogram all the software on my computer". The fourth freedom is needed to understand the value of this freedom.
The third freedom: "The freedom to redistribute copies". Sharing software is useful and simple, denying it for some perceived social gain based on the profits of one company is absurd.
The fourth freedom: "The freedom to improve the program, and release your improvements to the public". The job of making software do what it's users want is too big for any one person or company. Only by being permitted to collaborate will the job get done.
In the 60s, all software was free software. In the 70s, that culture died. In 1983, a guy called Richard Stallman decided it had to be brought back. For more info, you can read the full story of free software, or the short history of free software.
Since 1998, some people began calling free software "open-source software", so you may have heard of free software by that name, but I recommend that people don't use that term. The goal of the "open source" people was to escape the ambiguity of the word "free", but I believe the plan has backfired. For an essay about the two terms, see "Why ``Free Software'' is better than ``Open Source''". My thoughts on the matter aren't identical to those of that essay, but the differences of my opinion aren't enough to motivate me to write my own essay right now, so that one will do.
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