I've discovered Thunderbird saved searches. They allow me to create items in the tree thing on the left that act like folders full of messages that meet certain search characteristics. For example, I can have an item called 'fred' containing all messages from 'fred'. Previously to do this I would create a folder and get my filtering rules to try to shuffle incoming messages into the right folder. In outlook this was always flaky, in thunderbird I've never bothered. Saved searches are much easier to set up and have the advantage that if you change the search terms then the changes are applied immediately, you don't have to run all your messages through filtering rules again. Also messages can meet more than one search pattern: I could have a search for everyone in project X and I could also have searches for specific people in project X: the same messages could appear in both.
It's working nicely with Microsoft Exchange via IMAP: the searches occur instantly, no real overhead. This may be because the IMAP is offloading the search to the server. If the search was done locally I doubt that it would be so transparent.
Hum, as a concept these stored searches could be considered an alternative to tagging. Instead of manually having to mark an article as being about, say, ubuntu, the stored search would automatically search for the word ubuntu in the article and list the matches. It is more fiddly to create a search than a tag but they would require less maintenance. Tagging does give the possibility of structuring articles. Something else to think about.
Peter's Moblog
I also discovered this recently. I've been putting emails into project folders like I always have, then I've setup search folders for particular people. Works fairly well. However I really really wish they had tagging. Search only TRIES to pick out the concept or word you want, but tagging is intentional meta data and thus much, much more accurate.
I hope this comes soon.