Utility libraries, because I actually get to see my life improve because I have them. Creating new projects becomes easier because I put parts I reuse somewhere else.

There was an old config file generator/manager I kept using for a while, some string conversion libraries between formats, some REST/WebRTC API wrappers, I have a web audio API I create tunes with in various projects. CI/CD scripts for laziness so I never have to know how to set anything up again. Lately the thing I'm most happy about is I turned some free text saving service into a makeshift database and it's been working well for about half a dozen projects now. Wouldn't handle large amount of users but can't beat free and easy.

I also find merit in prototypes/old projects, because I can reference random things I did in them in newer unrelated projects. Things too small to warrant their own utility library, argh!