Tasks Jr. is an open source to-do-list application that works web-based, has also a PDA interface & built with PHP/MySQL.
It enables you to create tasks with sub-tasks where they can have different due dates or priorities. The application has an option to remind the upcoming tasks daily via e-mail.
Tasks can be sorted easily & a “search feature” helps finding them with keywords. It also has iCalendar support that makes sharing the tasks possible.
Tasks Jr. is multilingual & already translated to various languages.
P.S. The application is a light version of a payed application. Although not developed any more, it already offers the standard features & can be improved as it is open source.