Have been working on Lightpage for the past ~18 months. It's a personal notes app that "writes back" using LLMs.
Before this, I dumped all my personal notes, ideas, journal entries, etc into Apple Notes -- I liked the simplicity. But as LLMs get smarter, I wanted to have them read my notes and proactively help move things forward.
Right now most of the early users use it as a journal. On a whim I shipped a feature a few months ago where the LLM writes you a "letter" every Sunday based on the past week's notes. That's ended up being a really popular feature.
I've been hacking on a related side project -- in my case, I wanted something like this but for Twitter. Right now it's using gpt-3.5-turbo to cluster related Tweets & rank based on my interests.