Power your house with falling water:
Take two portals oriented one above the other. Place a Micro hydro generating plant between them. This diagram I found on the net illustrates the principle, although the system does not necessarily require 100 buckets of water (or other working fluid, for that matter; you could use bowling balls or sand if you wanted to, but water is probably easier).
This can be installed in many places where there is gravity. Depending on the energy requirements of the portal this may or may not be a perpetual motion machine. Also, there may be side-effects depending on the (fictional) characteristics of the portal.
HVAC the simple way:
Open a portal to a desired climate. Polar for air-conditioning, equatorial for heating. Install in your house with the portal bringing in cold dry air from the pole in the ceiling, and the portal connected to the tropics with warm, moist air in the floor. Many small portals are probably a better idea than one large one, simply for safety, although you will still want gratings to keep bugs in/out. Installing controls is probably a bright idea and/or you may produce interesting weather effects inside your home.
(From a conversation with digitallemur.)