Open Source software has formed the base of many successful and profitable businesses. Some of the advantages include:
Core software is free:
If you're just getting started in online business, cost can be a major factor. Using Open Source software can really cut down on your initial capital outlay. It's also my firm belief that the Open Source community has helped to rein in prices on commercial software over the years.
As mentioned, some Open Source software projects can have huge communities of programmers involved, allowing for the rapid implementation of new features and security fixes. The communities of users and programmers are also invaluable resources for asking questions relating to troubleshooting and suggesting enhancements.
Encourages hands on:
When you're short on cash, you are more than likely to want to make modifications to software yourself. I'm no programmer, but the use of Open Source software has encouraged me to go beyond the user interface; to dig into code to try and understand what it does and to make minor edits. As a business owner, it doesn't hurt to understand a little of the voodoo that goes on behind the scenes in the software you use on your site.
Not tied to a single vendor:
If you purchase a commercial application, you can then become reliant on a single company to solve your problems and maintain the software - which can also be very expensive. Some commercial software companies may only provide support and upgrades for a limited time before you need to fork out for any further enhancements or assistance.