I'm working on a system which allows people to get synchronized/coordinated reminders about anything on a daily basis. It's just a Rust library ATM, but I might make it into a website or an app eventually (or someone else can, since the algorithm is like 10 lines of code and in public domain). https://github.com/TypicalHog/randevu

What does this thing solve? Imagine a game (perhaps obscure/dead one) you and some other people really like, but there's never enough people playing it at the same time for it to function (imagine it's a multiplayer game). People could add the game to the list and the game would be featured to everyone who has it on their list of items on the same days. Let's say once a month or once a year etc. This would allow people to all come play/appreciate/discuss the game on the same days instead of each person randomly remembering it and playing it alone on random days when no one else is. The system also works for movies, videos, books, websites, people, servers and literally ANYTHING else. It also works offline with no internet connection. The system assigns each item a "significance" value each day. Values range from 0 to infinity and each value happens 2 times less often than the previous. For example: 0 happens every single day, 1 about every 2 days (on average), 2 every 4 days, 3 -> 8, 4 -> 16, 5 -> 32 and so on. So imagine a system assigned value 5 to some game today. Every single person who has set their "reminder/significance" threshold to 5 or below for that game will get a reminder about it. And if enough people do so - one can expect that game to have surges of activity every now and then (when its significance number is high). The system essentially "scoops up" everyone who's interested in something and directs them towards it on the same days. I'm sorry if this is confusing, but the system is actually super simple, I just might be bad at explaining it.

Here's a visual analogy:

Imagine if each object had a board like one in the picture. The system throws a dart randomly somewhere on the board for each object every day. The section where the dart lands determines the level of significance that's assigned to that object. Note that the dart lands on completely different place for different objects like MINECRAFT, XONOTIC and THE_MATRIX_1999. But, for each person using the system, the dart will land on the exact same spot for the same item. So if a dart lands on the section with the number 4 for the object THE_SIMPSONS - every single user who has THE_SIMPSONS on their list will "see" that the dart landed on 4 that day. And like I said - anyone who has their reminder threshold set to 0, 1, 2, 3 or 4 will have THE_SIMPSONS featured that day. Once again, the system isn't actually throwing virtual darts on a colorful board and is actually just spitting a single number for each item each day, but this visualization is IMO a good abstraction that explains the system.