Drupal and WordPress and the leading open source website content management systems.
They have large install bases, have many modules/plugins available to extend core functionality, and are straightforward to extend in bespoke ways to suit needs not addressed by existing modules/plugins.
Most websites we develop are developed using one of these two CMSs, extended as appropriate to suit client needs (these extensions range from simple image manipulation systems, to complicated shopping cart solutions). We have extensive experience in developing bespoke modules for Drupal in particular.
A typical Drupal or WordPress website we deploy will have about a dozen extra modules/plugins installed, delivering SEO functionality, image management, forms management, email marketing integration, social media sites integration etc, all in one cost-effective solution.