What apps have you created for your own use?

The one I use the most is probably https://developer-toolchest.com

I wrote a program that waits for a socket response. When I reboot a vm, I use it to know when I can ssh in:

https://github.com/brianmed/SocketFun/tree/main/socketwait

socketwait 192.168.1.10 22 --waitFor TcpRegexResponse --tcpRetries 300 --tcpRegexResponse '^SSH'

See https://news.ycombinator.com/item?id=38623695