Which program is the one that surprised you most that it is available on Linux?
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.
Kicad is up there with the paid options for electronic schematic drafting / PCB design. I don't use a lot of KDE stuff since I also don't use KDE, but Kicad is absolutely essential for me.
Despite the K in its name, KiCad is nothing to do with the KDE project. It's an independent program started (iirc) at a French university. I agree it's awesome, though.
It's so good that it absolutely killed my will to maintain a project I had that does something like this once I discovered it. They even support Windows too.
Out of curiosity, what is it you use it for? I pretty much only use it for SMS, for which it is kinda janky and unstable. Doesn't always get contact names, doesn't load everything from conversations, misses messages that I sent or that were sent to me, crashes if I scroll too fast, etc. I have Connect installed just to use SMS (cause I hate typing on my phone keyboard), but I'm honestly not even sure what the base software does.
If find the missed call notifications to be very helpful. I never remember to check my phone, but I'm on the computer all the time. I also like the ability to remote control the music player from my phone since the computer is hooked up to an amplifier with speakers in multiple rooms.
The media integration is great. I can comfortably pause videos I'm watching from my phone's lock screen. And it does it automatically when a phone call comes in.
And I can use it as a remote mouse and keyboard. That was the first thing I used it for when it was just a small app some guy created for their own needs.
And the clipboard synchronisation is also sometimes useful to get links or so from one device to the other.