In our house we heat with a heat pump (Wärmepumpe) which has an earth probe (Erdsonde).
The probe goes into the earth and takes heat from there and brings it into the house. So we use the earth's heat. It is actually a very environment friendly process, since we don't need any oil or something like that and it's renewable since the earth's heat is always active and doesn't get damaged within this process. But it's very complex since you need to drill the hole (ca. 120 - 150m deep) and put the earth probe inside and everything.
On this picture you can see how it works.
Also here's a picture of our heating room:
