IKEA Tradfri. Nice idea. Super frustrating to setup. Took over 10 attempts to pair a light to a switch. Ended up resetting the pairing of the switch, and it worked immediately. Of course, I paired the switch just about 20 minutes prior.
Oh, and then there was the firmware bug that prevented it from getting a DHCP lease from my OpenBSD router. Had to setup another network with some Mikrotik kit I had around to get it online and updated. Now it works on my main network.