Well, don't tell anyone, but I used to use Outlook with Business Contact Manager. It was great. When I exchanged email with a potential lead or a customer, I could log all correspondence from within my mail app. I didn't have to worry about remembering to forward to some address or have to manually enter the message on some 3rd party site.

I was give a simple "log message" button and it got added to the contact's history. I could optionally annotate it. But it was dirt simple for keeping track of email communication (far and away my most common form of communication).