I've got a few of these.

There's one in particular that I use quite often and have for about a year, vibed for myself: it's a chat interface that walks you through processing an emotional or difficult moment, following a process / workflow. Supports Martin Seligman's ABCDE and Byron Katie's "The Work". Two techniques that I found most useful to improve my thinking and responses to difficult situations. You converse with it, and it leads you through the stages of the selected flow. More than a system prompt - it tracks and progresses through the workflow at the right times, and you download a consistent pdf of key details from the "session" at the end for your records.

The thing is, I can't sell it. I don't even think I can open source it. It's mental health (minefield of legal and ethical issues), and would probably breach copyright, trademark etc law.

Yet it's very useful to me. I guess it's the equivalent of making a system to do these things at home, like prompt cards or key points taken from the books, with a well formatted note taking structure. Except it's way easier because you just chat like you're talking to someone.

I would never have spent time building this. But now it's so cheap and easy to do.