The following article was taken from this link.

Open source software is generally free software that you can use in your business. Open source developers choose to make the source code of their software publicly available for the good of the community and to publish their software with an open source license – meaning that other developers can see how it works and add to it. Examples of open source products include Open Office, the internet browser Mozilla Firefox, Wikipedia, the GNU/Linux operating system and its derivative Android, an operating system for mobile devices.

How does it work?

From a business user perspective, open source software works in much the same way as proprietary software systems provided by commercial software firms – the only difference being that generally you don’t pay for it. However there are a few important differences – the idea behind open source software is that users are effectively co-developers, suggesting ways to improve it and helping to hunt out bugs and problems. This means that if you wish, you can modify it to your own needs, port it to new operating systems and share it with others.

What are the advantages of open source software?

1. It’s generally free – it has been estimated that open source software collectively saves businesses $60 billion a year. These days for virtually every paid for proprietary software system you will find an open source version.

2. It’s continually evolving in real time as developers add to it and modify it, which means it can be better quality and more secure and less prone to bugs than proprietary systems, because it has so many users poring over it and weeding out problems.

3. Using open source software also means you are not locked into using a particular vendor’s system that only work with their other systems.

4. You can modify and adapt open source software for your own business requirements, something that is not possible with proprietary systems.

Any disadvantages?

1. Because there is no requirement to create a commercial product that will sell and generate money, open source software can tend to evolve more in line with developers’ wishes than the needs of the end user.

2. For the same reason, they can be less “user-friendly” and not as easy to use because less attention is paid to developing the user interface.

3. There may also be less support available for when things go wrong – open source software tends to rely on its community of users to respond to and fix problems.

4. Although the open source software itself is mostly free, there may still be some indirect costs involved, such as paying for external support.

5. Although having an open system means that there are many people identifying bugs and fixing them, it also means that malicious users can potentially view it and exploit any vulnerabilities.

The practicalities

You can download open source software onto your computer system in the same way you would proprietary software. Some software providers such as Alfresco, MySQL and Ingres offer both open source versions of their software and paid-for proprietary versions.

For a list of free GIS Softwares look at this blog post.