The latest Thunderbird release introduces some pretty slick virtual folders. They're fast too. Very fast. I've used them in two other email applications The Bat and Opera but Opera has always been the leader here with its M2 mail application. In M2 everything is a view on an aggregate email database and all folders are canned searches. It would also provide some dynamically generated folders, such as the last 10 people who sent you a message or the last 10 threads you replied to. Cool stuff. The Thunderbird implementation isn't that sophisticated but it does let you save a search as a virtual folder. I've started creating a who/what/when/where folder structure to test it out. Under the who folder I have a folder for each person I'm interested in, the search terms being that person's name as the recipient of sender of the message. The when folders have date ranges as criteria, whereas the what folders group mailing lists into subject folders. The where axis is less useful - so far it's just where messages were sent to, i.e. what email alias. Now I want to integrate to weblog postings and documents on my hard disk. Not sure how to do that yet...