There are a number of programs you can download and use that replicate the functionality of expenses proprietary software. For example, Open Office is an office suite which replicates Microsoft’s Office suite, which is expensive. Take Microsoft Word for example. The functionality of a word processor at a development plateau. There is very little value added work that a software company can do. There is little research that is required today to develop a word processing software.
Some free packages also expands what is available. Firefox is a free web browser which replicates and expands upon the functionality of Internet Explorer.
Operating systems can be open source. For example, Linux, which does not require you to pay license fees.
A good starting point for information about open source software is Open Source Initiative
They have a centralised list of most open source software licenses. License categories are available to download here
There is a distinction between what users write as a human and the file that the computer understands.
One of the most common licenses is the GPL. Many people can add their own contributions under the one software license. Hence that body of software will grow.
Apple’s Mac OS X core, Darwin, is a free and open source operating system (OS) released under the Apple Public Source Licence. In addition to this core, Apple bundles various open source software as part of their standard Mac OS X release including MySQL, an open source database and Apache HTTP Server, an open source HTTP server for various operating systems such as Mac OS X and Windows XP.
Apple also has a page dedicated to Open source for developers using Mac OS X.
Apple provides a page dedicated to open source software downloads available for Mac OS X. Alternatively, a simple search on the Internet can reveal websites dedicated to providing a list of open source software for Mac OS X ranging from web browsers, media players to emulators which allow Windows to be run within the Mac OS X environment. Open Source Mac is an example of such a website.