One of the features we built into the Outlook 2003 user interface is called Quick Flags.
You can single-click next to any mail item to color it so that you can find it again later. Or, you can right-click to apply one of six colors to help you categorize it further. Either way, all marked messages automatically and immediately appear in the For Follow Up search folder.
This feature has become quite popular and became the impetus for the redesigned categorization and time management system being introduced in Outlook 2007, in which the same colored categories work across all parts of Outlook.
Probably the single biggest feature request in this area has been to enable the naming of Quick Flag colors. This is totally built-in to Outlook 2007, but as you can't go purchase that just yet, I wanted to show you a cool trick which gives you something close in Outlook 2003.
The article describing the trick is in found on the Office Online web site: Name Your Colored Quick Flags.
It's not a complete panacea (it doesn't change the context menu, for instance) but it does provide a useful, interactive legend as to what each color represents.
This tip works only in Outlook 2003, because Quick Flags (and search folders, for that matter) didn't exist in previous versions.