That is a great question! There are two kinds of plants that use alternative energy than the sun. There are Saprophytic and Parasitic plants. Saprophytes are plants that take their energy from decomposing material in the soil that is rich in nutrients and energy. The decomposition process occurs when fungi, bacteria and other microorganisms break down plant and animal material that dies and lands on the soil surface. Saprophytes take advantage of this process and suck up all of the good stuff that is released by the decomposition process. This is a really cool saprophyte called "Indian Pipe" :

Parasitic plants will take nutrients and energy from other plants or even fungi. They steal nutrients and sugars from plant roots, stems and sometimes fungi. A common parasitic plant around here is called Dodder or Witches Hair and looks like this:

It is the orange stringy plant with small white flowers. You can find these plants in the salt marshes around Santa Barbara. Look for orange spaghetti-like stuff. This plant in particular has structures called haustoria that puncture the victim plant (like vampire teeth) and suck out the good stuff inside. Plants are super cool!

Non photosynthetic plants are usually parasites, most commonly on other (photosynthetic) plants. They get their energy from the hosts that they parasitize.