[smarterchild]

You could put that together with this: http://3mindme.com/

Say: "I would like an email back about this email in 1 week."

Disadvantages to this:

* Your email goes out to them and gets stored for that long.

* You have to remember to include it as a bcc on every email you care about following up with.

* It doesn't automatically snooze if you get a followup.

* You have no way of tracking responses (in, say, a DB).

But it's a cheap solution that will do the job. Otherwise, there are special CRM packages for this. Unfortunately, I don't know much about them.