There are a lot of email tracking tools out there, but I really, really love the user interface of FollowUp.cc. It's a great tool to use in Gmail and I like being able to track all emails.

I appreciate how quickly I can send an email to followup and have it read...so if I need to take care of something in a week, instead of having an email sitting unread for the week, i can just receive it again in 1 week...knowing I won't miss it or it getting buried whlie also being able ot postpone it being unread