I've used MS Outlook for a few years now, as part of my GTD-ish system for tracking tasks, appointments, and of course email messages. After quite a while of using a tricked-out routine using some variations on the built-in outlook categories (those colored dots that outlook lets you tack onto any message, task or appointment), along with the usual gazillions-of-folders for projects, customers, billing, personal messages and every other possible categorization of outlook info - all managed by clicking and dragging messages and tasks here and there - I went looking for a better solution... and I am so glad I did.

Enter "Clear Context" from http://www.clearcontext.com/ - this little add-in makes such a big difference to my day. While it boasts a long list of features ( see the user guide, here: http://www.clearcontext.com/user_guide/ ), I'll touch briefly on the ones I use the most, and let you see for yourself why i am so excited about this tool.

1) Folders as 'projects'. ClearContext analyzes your Outlook folders and uses those as the basis for what it calls 'projects'. The entire application runs based on those folders - replacing outlook's somewhat limited category options with a limitless number of options for filing. When i get a new client or project, i create a folder, and once i've put a message from a client in that folder, CC 'suggests' that future messages from the same person go there as well. Unlike regular message folders, these same project/folder names are applied to your tasks, and CC's easy 'dashboard' view lets me see all messages, tasks, attached files and contact information for anything in that folder or related to that 'project'.

2) Put stuff where it goes with keyboard shortcuts, and smart suggestions. With any message highlighted, i can hit ctrl+shift+m for an intuitive list of likely folders where this message belongs (ClearContext gets to know who you put where and suggests it back to you, becoming more accurate as time goes on). So, without taking my hands off the keys, i can file an entire day's worth of email in a matter of seconds. This is big, and the way it works is super slick.

3) Defer / Unsubscribe. These are two separate features but both of them serve the same purpose - getting things out of your face that you don't want to see! With a single click i can 'defer' any message for a set number of hours or days, or until a set time and date - it is removed from view, and at the set time, it comes back to my inbox as a new message. So, if stuff piles in for 'later' while i am working on correspondence for 'now', i can defer everything else until i am ready to deal with it, and don't even have to look at it until then. 'Unsubscribe' uses the message subject to create a rule which puts all messages using that same subject line into a special folder 'clear context deferred', which i look at and then empty to deleted items every few days. This is also huge - repeat emails, junk alerts, group messages where i am bcc'd but really don't need to be - they all go away, never to be seen, clicked or managed again (unless i want to ... then they are easy to find) 

4) Tasks from messages. More keyboard shortcuts, or a click on a single toolbar icon, allows me to create an outlook 'task' from any message. Even if I file or delete the email message (which i often do, once a task is made) , CC saves a task with the due date i specify, and the email text as the body of the message, with the original message attached. This makes it uber-easy to look through my to-do list , by category, and see what needs to be done. If i mark the task complete, or delete it, the original message is still where i put it, for future reference, without the need to refile it from 'to do' to the project it belongs to. This is incredibly useful for the way i work, and the way i use Outlook.

5) Do not disturb. Turns off auto send/receive for a limited time, and hides the 'you have mail' icon along with any sounds related to same. Click that, leave me alone, I am working.

6) Follow-up. When sending any message, there's an easy option to 'follow up'. This creates an automated alert, again at a time and/or date i specify, which is automatically cancelled when somebody replies to the message (lots of options there for who must reply to cancel the follow-up, etc) . If i send off a proposal (or even more importantly, a bill) and want to make sure the client receives it , I simply click the follow-up button and can put the whole thing out of mind. If the client replies, the alert is cancelled - if they don't, i get reminded to follow up. Super simple, super useful. 

Overall, my goal is to keep things organized and my inbox empty so i don't have to think about any of it. By making easy to file and find all email messages, tasks and other important 'to do' data, I can free my 'mental RAM' from carrying those things around, and focus on whatever i am doing at the time. (Now, what was I doing... ?)

If you use Outlook's tasks, or file your messages by folder, or just like to keep your inbox clean, I suggest you take a test drive (30 day free trial, $89.95 to buy w/ money-back guarantee) of this super useful, well-maintained and carefully implemented add-in. 
You can also check in with other users in the active, well-supported Clear Context forums: http://online.clearcontext.com/forums/  to find how it can help you get more done.