I'm a big fan of NEO to make Outlook's Inbox more GTD-able.

NEO is a meta-Outlook. It adds some key views & new concepts on top of Outlook's email folders. Here's what makes it great:

Search

NEO transparently searches all the .PSTs you want -- including your Archive folders. So I now auto-archive weekly and don't care about where things are. No filing means, um well, no filing.

Active Mail

They add an "Active" Mail concept. This is a new view of potentially all of your mail in all of your folders. But if you don't need a mail, hit BACKSPACE and it goes inactive, and thus out of this main view. Buh-bye. Won't miss ya.

There are also rules & training to keep some items from never showing up in your Active Mail. Bulk mail folders, for example, for mailing lists, etc.

This allows for some good Inbox Zero practices. As long as I practice one or two email dashes a day, I can keep my Active Mail close to empty. But I'm getting better.

Categorization

There is also drag & drop categorization in a sub-view of Active Mail. So I've set up Waiting, @Work-PC, and Reading List categories for sorting email items per DavidCo. Then I purge through those categories as needed.

This is slightly different than Outlook folders in that I don't care where Outlook puts them, but I do care about keeping my Active Mail empty. Once I've processed the mail, I de-categorize it and then it drops out of @Work-PC (or whatever).

I guess this is harder to explain than it is to use. They have a free version, but I ponied up the dough as I was hooked last year. Meanwhile, I've recently picked up GTD and realized that with only one key - backspace - Inbox Zero is within my reach.