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.
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.