Thanks to everyone who provided comments and feedback in my first Outlook as Organizer post. In my first response to comments from that post, I'm going to explain how you can set up Outlook to organize your email in the same way that GMail labels work. In the GMail system, instead of filing your mail in a folder, you just label it. You can then view all mail with a particular label easily. The main benefit over folders is that a message can have multiple labels, whereas you can really only put a message in one folder.
To set up a similar organizational system in Outlook 2003, we can use categories and search folders. Just like labels in GMail, you can have multiple categories on an Outlook message (or any Outlook item). To set categories on messages you can go to Edit | Categories, or right click on the item to get it from the context menu. I like to use the keyboard, so I often hit the <menu> key and then "i". In the categories dialog you can set the categories for the message or go ahead and edit the master categories list.
But assigning categories isn't enough. You'll also need to set up search folders. For each category you use, create a new custom search folder. You'll set the criteria by changing the Categories text on the "More Choices" tab to the category you want to view in this folder. Whenever you create a new category, also create the corresponding search folder (which takes all of about a minute) and you're good to go.
In addition to the basic GMail functionality that this organizing scheme creates, you also get all the great benefits of Outlook. You can change the view in each search folder to make sense for the messages you're looking at. For example, you can arrange messages in a "Personal" search folder by conversation, and those in a "Project X" by who they're from, and those in "Project Y" by date.
I'm currently working to file all of my mail this way. However, to make it even easier, I'm setting up some search folders by contact, to show me all the mail to and from a certain person. I don't have to do anything to organize this mail - it does it by itself.
UPDATE: Looks like someone went and made it even easier.