I made a frontend for nntp. Even if usenet is long dead.
I was following a group (it.comp.console) at the time using thunderbird but I didn't like google groups so I wrote my own nntp web frontent to follow the group on my smartphone. I contacted some usenet server admins and asked them if I could peer with them (one here on hn), so I set up an nntp server (inn2) and wrote a node/express/js app to read and post to the group.
I make zero money from it, but people liked it and started using it, and now i have about 100 users on the website. I made it mainly for myself as a hobby, but I like that some people like what I made.