I just discovered the "Get mail from other accounts" option in GMail. Enter the account name, password, and POP3/SMTP/IMAP info, click the link in the confirmation e-mails, and all of your mail comes through your GMail account. Hit "compose", and the "From" field becomes a drop down with every e-mail address you have.
My new job uses a third party e-mail service, so I just rolled my personal, KxK, and work accounts all into one. Then I set up labels to sort everything for me. If I could just get my boss to use Google Calendar, I'd be able to manage my job, band, and personal engagements all from there as well.
and I've used Outlook Express for years now. Does the same thing.