Earlier quoted context omitted.
Boomerang for Gmail [0] has implemented this functionality well. I prefer it to say having an entire folder of emails I haven't received replies on as it allows me to specifically manage messages that are important enough to require a non-replied reminder. [0] http://www.boomeranggmail.com/
Boomerang and Followup.cc are both great alternatives, and I use them too. I like the granularity, but the downside is you're sending them a copy of your email, which might contain sensitive info :\
It's written in Go; there's no external dependency except a service that downloads emails to a Mailbox-type directory (I recommend offlineimap).
I've been dogfooding it myself for the last few months - I haven't actually "released" it officially yet, but if anybody is interested in beta testing it, I'd love some feedback.