Ethiopian volcano Erta' Ale' (Pronounced "Er-tah-All-Ey") is classified as constantly active, but it seldom produces very powerful eruptions.

Most of its observed life (since 1960), it has consisted of a "static lava lake" with small eruptions (only 2 on a scale of 10) like the one in this video:

Erta' Ale' is over an area where the earth's crust is diverging and the mantle (the very hot layer below the crust) is directly exposed at the surface. The theory about its lack of eruptive power is because the magma/lava is so close to the surface that it doesn't have a chance to pick up "volatiles" (such as water which generates superheated steam) needed to create more powerful eruptions.

The analogy is often made that Erta' Ale' is like a muscle car with only a teaspoon of gas in the tank.

 

Sounds lik a good geothermal source of energy.

 

Many volcanic regions do have geothermal plants for power and building heating and Iceland is the largest user of geothermal energy.

They do not drill directly into a volcano as this can trigger a release of lava. However the area several miles off to the side of an actual volcanic zone contains hot "magma" which is a "quasi-solid" material that won't flow and turn into lava.

Injection and extraction (steam) wells can be drilled in a magma field without risk of an uncontrolled eruption or melt down of the piping.

 

This is a recent video of Erta Ale which shows the size of the lava pool relative to people walking around the area.

It seems Erta Ale is the most stable volcanoes on Earth.