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.
What surprised me the most, also in part due to me not really being knowledgeable about software solutions in their respective industries, was the Unreal Engine (the editor that is) and Houdini being available on Linux. Tbf, at least in the vfx department it is apparently more common as most of the high profile software in that industry does have a native Linux version available.
What I appreciated the most though was software like Reaper and Renoise providing a (very good even) Linux-native version when I looked for a new DAW to learn, seeing most software in the audio industry not being very Linux-friendly.