For me, it was perhaps simple-scan, a very simple and efficient GUI to scan documents. I used it with my Brother printer / scanner and it works like a charm. Especially since I do not scan stuff often, so a program with more complex UI would have the effect that I forget how to use it until the next time.
Surprised no one has mentioned OBS. I don’t use it for streaming, but afaik it’s one of the more popular options for that. So it’s really cool that not only is it available for linux, but it’s open source and works great. I’m sure every linux user has had audio, general hardware, or GPU acceleration issues at some point, but OBS is seamless in my experience. Pretty cool to see a piece of software live at the crossroads of all that and get it right.
I believe it was the Linux counterpart of XSplit, the “other” streaming software that was very popular in the early days of streaming. Before that we had to use some very elaborate setups.