Google have just added a to-do list feature called "Tasks" to Gmail. It has an easy-to-use interface, particularly using keyboard shortcuts:
- Indent - Tab
- Un-indent - Shift + Tab
- Move up - Ctrl + Up
- Move down - Ctrl + Down
- Edit details (set due date and edit additional notes) - Shift + Enter
To enter a task just click in an empty part of the list. To change a task just click on it for in-situ editing. You can also convert emails into tasks.
Tasks is currently only available in Gmail Labs. For more details, go to the Gmail Blog.