Although not open source software, ExpressionEngine has many ways in which both EllisLab and the members of the ExpressionEngine community can add to it. ExpressionEngine 2 is built on CodeIgnitor — an open source PHP web application framework also by EllisLab. CodeIgnitor is a popular framework for PHP developers, allowing rapid development of PHP web applications. For ExpressionEngine users, this means that add-on developers have the ability to build more complex add-ons (even entire applications) for ExpressionEngine by using CodeIgnitor. It also means that there is a much larger base of developers who can build ExpressionEngine add-ons.
More developers building more and more complex add-ons for ExpressionEngine can only mean one thing. If there is something you need to do that is not supported natively, there is likely an add-on for it.
The add-on market has grown hugely in the last few years and there are now several companies that have either started or been...