One of the most critical decisions for a company to make when they are launching a website is to determine if they need to use "custom code" for their website. Obviously, if you are just going to have a simple website with a few pages about your company and services than odds are you do not need custom coding.
What is open source? Open Source refers to software that allows free distributed and access to the product source code.
There are literally thousands of applications that are "open source" and thousands more programmers and customization houses (like Creative Domains) that will take open source framework and create custom modules and components that create incredible capabilities at a fraction of the cost of developing a program from scratch.
Obviously, there are programming requirements that simply can't be met by using open source applications but it is always a good starting point to see what is available.