Loving Oasys Mail Manager. I know it's really designed for collaborative working, but this sole trader could not live without it now! These days I can find any email and its attachments in seconds.
http://www.globalbankingandfinance.com/News/Social-mapping-of-email-shows-how-the-team-works.html
http://www.globalbankingandfinance.com/News/Social-mapping-of-email-shows-how-the-team-works.html