I have been a hobbyist web developer since the 1990's (I did it professionally then), and I regularly build websites to fill gaps in existing services.  Most are just for my personal use, but two have matured to the point to become generally available, and are likely useful to other academics.

Gradebird

This one started in ~2021 when a colleague, Drew Milsom, told me about a "self-assessment" workflow he had been using in his courses. The idea is that each homework assignment comes with an additional, post-completion step where students assesses their performance based on posted solutions. I thought it was a brilliant way to get the students to actually learn from their mistakes, but I didn't like all the overhead associated with the additional submission and grading steps--traditional learning management systems just aren't built for it. So I made my own web app, which eventually turned into Gradebird. After use by multiple instructors in courses of all sizes here at the University of Arizona, this app is battle-tested and ready for more general use. Shoot me an email if you would like to try it out yourself.

MagicForward

Like many academics, I forward my university email to a personal gmail account so that I don't have to check two inboxes. This worked just fine... until it didn't. I was tipped off in 2022, when I received an email from an editor of a prestigious scientific journal at my personal gmail address. This is rare since professional correspondence generally goes through the official university email, but the editor was a friend who knew my personal address. He told me that he had been trying to contact me at my university address and hadn't heard back. I was quite surprised because I am very good at answering email. I even check my spam folder regularly. His messages were nowhere to be found! After a long period of sniffing around, I discovered what was happening: gmail was silently rejecting a subset of messages for some rather complicated and sadly unfixable reasons (see details here.) The only solution is to forward to a server that I control, accept every forwarded message, and the copy it to my inbox using IMAP. I set this up for myself and now offer it as a service at MagicForward. It's publicly available for signup, so you don't have to contact me to use it.