FollowUpThen

You can do a lot with this software depending on how much your willing to pay and let it have access to your emails

Basic features

• Can ping a reminder back to yourself at any specified amount of time
• Can set a deadman's switch to constantly remind you of a task until you tell it not too

Advanced features (forsaking privacy for more functionality)

• Can tell you what items were not replied to, if you give it access to your inbox. Gmail + googlelabs has something similar, but nothing existed for thunderbird users

Privacy features

• You can tell it to only received subject lines / from / to / date metadata only (no body information and or attachments)

Free version is limited to so many pinged reminder emails a month though, but should suffice for most people