The Webbler is a content management system written in PHP, almost entirely by Michiel Dethmers, a former business partner of Tincan Ltd. From about 2001 until 2011, Tincan Ltd provided advanced complex websites to its (40 - 50) clients, including the National Theatre, using the Webbler.
The Webbler is a modular CMS and most of its functionality is provided using plugins. The core code consists of some 200.000 lines of code and the total of plugins written for it are another 200.000 lines of code. It is designed to handle websites hosted on a cluster or machines, and it happily handles a million visitors per month on a cluster of 3 to 5 reasonably specced machines, serving personalised pages (eg when the visitor has logged in to the site).
In 2011, Michiel left Tincan to dedicate himself to phpList, which had been an on-the-side hobby project until then. He offered to continue to maintain the Webbler, but Tincan declined the offer and dropped all efforts to maintain the Webbler. It has been trying to convert the Webbler clients to Drupal ever since.
Michiel continued to maintain his own version of the Webbler for the phpList website, which is the only properly maintained branch of the application. The Tincan version of the Webbler was grinded to a halt on November 8th, 2011. Nevertheless, Tincan continues to host its clients on the outdated and poorly maintained software, exposing itself to increasing problems of maintenance, and most importantly security.
If you are interested to know more about the Webbler, contact the phpList team.