As developers we normally have a ton of open windows: your terminal, code editor, web browser with 8343 tabs, Spotify, Slack, etc. Magnet is the missing window manager for mac os that allows you to use keep your workspace organized. If you are coming from Windows this is a must. You can easily arrange windows by dragging to various points on the screen or by mastering the keyboard shortcuts. This is by far the most important app on my mac.
CopyClip is a clipboard manager that lives in your menu bar. It allows you to see your copy and paste history so that you don't have to keep going back to StackOverFlow to copy and paste that one piece of code. This is a huge time saver for me.
iTerm is a feature rich terminal replacement app. It looks amazing and gives you the ability to have multiple terminal tabs within the same window. Another fantastic feature is the ability to search natively within the terminal. Coupled with OhMyZsh, I think it is the best looking terminal app.
If you work with NPM at all you know how large some node_modules folders can get. npkill will search though your file system and display all of you node_modules folders along with their sizes. It then allows you to easily select any that you want to delete. This can help you save GBs of space on your hard drive.
Notion might not be a "lesser known" app, but I am truly surprised more people haven't heard of it before. Notion is such a powerful tool with so many different features it would take me forever to explain them all. I personally use Notion to keep all of my notes, reading list, and side project issue tracking.
If Notion is a little too feature rich Notable is a great simple markdown editor. It is great for quick notes.
If you are using xCode at all and are running low on space on your mac this is a must. It will scan your computer for cached xCode files and lets you easily delete them. This app has saved me 15gb of data.
Technically this isn't mac only but still it is a great tool that not many people know about. Synergy allows to share a single mouse and keyboard with different computers. I personally have bee using this with my work for home setup. I can use my mouse & keyboard plugged into my windows machine and seamlessly move over to my mac without having to use a physically KVM. Even though this might have a high price tag, I couldn't recommend it enough.