Toodledo

"An easy to use, web-based to-do list. Get organized, stay motivated, and be more productive."

http://www.toodledo.com/index.php

Todo,txt

"Countless software applications and web sites were built to manage your to do list. But if you're comfortable at the command line and you don't want to depend on someone else's data format or someone else's server, there's an age-old method that's perfect for tracking your stuff: plain text.

Keep on top of all your tasks and projects in a simple file called todo.txt. With a few helper scripts, you can slice, dice, sort, distribute, pipe and munge your lists any way you please. As it should be."

http://todotxt.com/

Simple todo list and task manager: Todoist

"Todoist is a popular online task manager that's useful, fast and easy to use."

http://todoist.com/

Taskfreak

TaskFreak! is a simple but efficient web based task manager written in PHP.

Todoist - Simple todo list and task manager

http://todoist.com/