I have started experimenting with EnOcean devices to add to my home automation system.
The sensor kit and the Raspberry Pi interface were quite cheap, but the finished devices are usually expensive as they are mainly aimed at the commercial market.
The sensor kit includes a temperature sensor, a magnetic contact switch and a wall switch.
All the devices are wireless (868Mhz ASK) and energy harvesting, so they need no batteries or other power sources and are maintenance free. The temperature sensor and magnetic contact switch have solar panels and the wall switch is powered by just pushing the switch.
I have the C examples running on a Raspberry Pi and can read the incoming values. I will probably write a C socket server to work with my home automation system. I could use Java, but as I probably cannot put the Pi interface on the same Raspberry Pi as my HouseControl Java system (which already has multiple wireless transceivers) , there is not much point and C is simpler. HouseControl can be configured to use any number of remote socket servers.
If the contact switches were cheap enough I would put them on all my doors, windows, letter box, fridge etc.
I will use the temperature sensor in one of my rooms. Someone has created a 3D printer model for it, so I will try that. It has an optional plug-in humidity sensor.
The other sensors could do with 3D printed cases too.
I cannot currently think of any sensible uses for the wall switch, but silly ones come to mind.
OpenRemote supports EnOcean, so I might try that too, although I am not sure if it supports the Pi interface.