ackerTodo is a web-based todo list manager which supports multiple users, recurring tasks, multiple languages, categories and much more! It's written in PHP/MySQL.
SPM for XOOPS is a set of XOOPS modules (collaboration management, document management, tasks organizer, events calendar, etc) to manage projects and all related tasks with them.
php todolist is a more advanced approach for simple todo lists. It is an ajax/sql-based multi user todolist, with priority, status and categories. You can use it as a bug track / feature request and task manager system within a group of developers.