You're pinging somebody over email right now, but they'll probably need a reminder or follow-up in two days. CC [email protected], and if that person hasn't responded yet, they'll be automatically re-pinged 48 hours from now. It's a neat service, and it's free.
That's the main way FollowUpThen works: CC a message that needs a second push to an address like 5hours, 2days, 1week, or even [email protected], and if your recipient doesn't reply (with FollowUpThen as one of the addressees) before your time period is up, the original message is re-pushed by FollowUpThen.com, with a little email graphical tweak, seen above, and a notice that FollowUpThen is acting on your wishes.
If you want to handle the second notice yourself, add the @followupthen.com timed address to the BCC field, and it will only get back to you, with a reminder of what you sent. You can also send a message directly To: followupthen.com, and your reminder is pushed back to you at the time you specify.
No software installs required, and it works from pretty much any email client after you verify your address upon first use. If you've got a clever use case for FollowUpThen involving webapps or other tools beyond the standard person-to-person email, tell us about them in the comments.