If you are one of those people (like I am) who likes to work from a task list, you will want to check out Google Tasks. Google Tasks is a sub-component of Gmail. It is by far the simplest task organizer I have ever used. Just start typing to add a task, press enter to add a new task and press tab to make a sub-task. Tasks can also be rearranged on the fly.
If you are looking for a simple task keeper to help you manage your day, Google Task is worth checking out. To enable Google Tasks, click Settings from your Gmail page and then click on the Labs tab. Then, enable Tasks. The tasks link will appear on the left of the Gmail page. You can pop-out the task window to create a standalone task page that is always easily accessible or you can embed Google Tasks within your iGoogle home page.
NOTE: This feature is also available to Google Apps users.