Followup.cc might be a bit more popular than Portal. We know about 1 link to it since March 2021 and only 1 link to Portal. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
There's nothing to buy, it's free and already included with Portal 2. Source: over 2 years ago
I'll update this if I find more. For now, this seems to be a partial solution. It's a Gmail add on (except I don't use Gmail) that adds reminders. The options when setting reminders include cancelling it if someone replies, and to remind everyone in the thread. If you combine those two, it seems like — if you use Gmail's web client and have $23/month you're dying to get rid of — a decent way to add the feature. Source: over 1 year ago
Antichamber - "A game with half of the attention to detail and imagination Antichamber has would be a smash hit - this bare-bones first-person puzzler is one of the most thought-provoking, engaging, and challenging video games ever developed.
FollowUpThen - The simplest way to schedule an email reminder
Quantum Conundrum - Quantum Conundrum is a First-person Perspective game with strong Sci-fi, Puzzle-Platform, and Single-player elements published by Square Enix and developed by Airtight Games.
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Mari0 - What happens when you mash Super Mario Bros. and Portal together?
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.