In recent years the argument for using open source products has become more compelling. In the arena of CMS this argument has been more hotly contested than virtually anywhere else.
Open source platforms such as Joomla, Drupal, Typo3 and even Wordpress have begun to be used more commonly in large corporate environments as decision makers begin to recognise that these products can stand shoulder to shoulder with their commercial rivals.
Lower costs.
While not a ground breaking notion, it is worth mentioning that, almost universally, the implementation, support and subsequent development of an open source platform will be significantly cheaper than a commercial one, even when discounting license fees from the equation. With OS products you are paying for the service, and not the software. This means that a significant proportion of your overall budget can go in to: customisation, integration, styling, training and more accurately meeting the business requirement.
Customisation.
The flexibility offered by open source is afforded by the ability to amend and adapt the core code. In the right hands this can be a powerful advantage. Though it must be said that careful control and planning is required to avoid costly "over development".
Access & Support.
Many open source products are supported by in international development community. Patches, updates and enhancements are generally both regular and robust. In addition to the core product support, specific support and development can be easily handed to an agency with the requisite skills without the complications of licensing and IP ownership.
Universally Robust.
The nature of open source means that the application is often required to perform in a huge number of diverse environments. The testing on such a variety of platforms and subsequent adjustments and fixes is virtually impossible to replicate using a commercial model.