Finally, Gmail has added a task list. It has been a much requested feature for a long time, not sure why they only got around to it now.
The task list is not enabled as standard, it is part of “Gmail Labs”. To find this, go into Settings and click on the Labs link. There you will have many advanced options – including a very handy “Forgotten Attachment Detector” which prevents you from accidentally sending messages without the relevant attachments. It prompts you if you mention attaching a file, but forgot to do so.