One of the first articles that I wrote was about Google's TASKS feature in Gmail. I was probably segueing away from using it at the time. Now I am back.
TASKS is part of the Gmail system whether you use a simple gmail account or a Google Apps account. Like Outlook and Apple Mail, TASKS gives you a place to list TO-DOs. You can start as many Task lists as you want so you can keep things in categories, or, perhaps, organized by project. This is how I think of things and how I use TASKS. I have projects for actual projects, but also Strategic Projects that I am working on for Architekwiki. A project might also be a set of reminders about invoicing or other 'overhead' activity.
What makes TASKS unique from anything I have ever used is its ability to connect an email conversation to a task. You simply select 'Add to Tasks' from the 'More' drop-down menu while you are reading the email or have it selected. The task name defaults to the Subject of the email, which I usually add to or replace with something more descriptive of what needs to be done. When you review your task list, you can click on 'related email' to recall the email. So the task listing is both a reminder that follow up is needed as well as the background reference itself. Clicking on "related email" brings up the original email along with all others in the thread/conversation no matter how old or where they are archived.
The unique "related email" feature doesn't come at the expense of all the standard features you need in a TO-DO system. TASKS has due dates, notes, sub-tasks (by indenting), sorting, printing, emailing the list, clearing and viewing completed tasks, etc.
Another advantage is that you can rely on TASKS to track critical emails without the effort of managing special "stars" or colored "flags" in your inbox. I have used the tactic of moving critical tasks to the top of the list (simply drag) and in some cases created a 'Critical' task list into which I move those tasks (also easy, two clicks).
The mobile apps for TASKS gives you access to your tasks no matter where you are, even though this is basically a browser-based tool. If you find that a lot of your TO-DOs and follow-up comes from email exchanges, this is a perfect way to simplify your system for tracking what needs to be done.