Some devices I've tried:
Sensi thermostat Wink spotter (no longer available) Dahua security cam
Lockitron was hopefully going to be used for access, but it didn't work out.
I've considered adding a tablet for guests to lookup instructions for the house.
I have an "EKM metering" (ekmmetering.com) gas meter that can (at some point) be connected to a pulse counter - they have a device that can tie your electric, water, and gas usage into a graph on the net (and they have an API).
Tried a wemo light switch - it sucks.
I have an Ambient Weather station that feeds current weather stats to weather underground.