I have been toying with email systems as long as I have been working on a PC and think I have found a really good system. It centers on Gmail and how it simply handles all the conversation in a recoverable thread. This natural and simple interface makes for uncluttered in boxes and good handle on the flow of information. Couple this with the excellend filtering and search capability and I have moved all my information and email flow to a system using Gmail ...
One use that I find particularly effective is using the email system as a master task and todo list along with a great tickler system ... let me describe the system, I first have started by setting up labels to function as my normal "folder" system ... I have several key labels, @tasks, @personal and @waiting ... then all sorts of others that I can "file" my emails ... the reason I use the @ sign is that the character will take the label to the top of the label list so it is easy to see things as they appear in the folder. I use my mail to send tasks to the folder ... I have set up a filter so any subject line containing "task" goes right to the folder and bypasses the inbox. I do virtually the same by filtering family emails to personal and taking requests for things and labeling @waiting. A quick and easy review of the filters makes an effective scan of the items ... the email of the tasks works like a charm and is very quick as I keep the Gmail program running all the time in my browser.
The final item is the Google Desktop search which indexes all the email and can be used even if you are offline ... amazingly useful and for those of you that have so many threads up in the air, it can be a lifesaver ... I can't tell you how many hours I used to spend trying to find a particular item or thought ... with this system it is almost instantaneous ... good luck with this and let me know if you do other things that work for you ...