There are lots of these, but this is the first that I've seen that focused on frontend dev a bit more. I've saved it to my list of tools for reference.

Here's another with a more local / backend / IT flavor: https://it-tools.tech

I have a couple more local apps with similar functions. Here's one that's cross platform[0]. This one appears to be Mac only[1].

Someone else mentioned not being able to remember these sites when needed. I recently started manually keeping track of web tools in html files inspired by a random repo[2] that fit well into a mode of category-abstraction that suited me. I don't recall how I landed there, but I liked the minimalism and adapted it to be a jumping-off point to a personal kbase that I made with another tool[3] some years ago. I have no design skills, so this (start-page) was just the right combo of minimalism and tasteful CSS for what I wanted. Works with markdown, which I also recently started using a lot more.

I ended up writing a lot more than I originally intended because I kept thinking of more links. They may be out of order because of non-linear editing and my having to rearrange them, so heads up. Also, it's early and I might just have made dumb mistakes.

0. https://devtoys.app 1. https://devutils.com 2. https://github.com/oinam/start 3. https://github.com/alanagoyal/docbase

Edit: Oh, looks like the it-tools link came from cruising the repo of start-page or vice-versa. Ha!