Nextcloud as the server and DAVx⁵ with Fossify Calendar as the client on my phone. On my laptop Kontact, but I haven’t looked at that in ages, so it could be in shambles for all I know. I think I’ve also used Thunderbird with some plugin.
And my Fritzbox router uses CardDAV to populate the phonebook of all connected phones.
My cousin programmed a simple labyrinth game on my C64 by randomly placing symbols on the screen and you had to get the cursor from the upper left corner to the lower right without bumping into a symbol.
I was so impressed.