A post on LifeHack today described a possibly more efficient variation of a Tickler file. Rather than sorting by day, they suggest sorting by action. I agree completely and just started using a similar approach last week. I use Outlook tasks with the Categories field, and a Group By in the view. The result looks like this:
It supports multiple categories (works the same as tagging), due dates, reminders and attachments. I can attach emails by reference or by value, and even "assign" the task to other via email if I need to. The copy machine also supports scanning documents to e-mail, which fits into the mix in perfectly. So-far it's working out quite nicely.