Rivers Cuomo has clearly been using his 2020 downtime wisely, as he is currently undergoing the final project for a course in web programming. As such, he's selling over 2,500 previously unreleased demos as part of this work…
The material – a grand total of 2,655 demos – is taken from all throughout his career, from unreleased Weezer ideas to work that was penned before the band even started (in fact, the first demo is from 1976, when Rivers was just six years old).
There's a few little caveats to these demos, so don't expect everything to be completely fully-formed. For example, the musician states that this material "might have silence", "might be wildly inappropriate", or may even just be Rivers "rambling, talking, making sounds" (which still sounds pretty great tbh).