Never put a portable heater on a countertop. It should be on the floor to minimize the danger of being knocked over and starting a fire. (The exceptions to this rule are models specifically designed for tabletop use.) Do not use an extension cord with a space heater. Most manufacturers provide a cord that's long enough to reach the power outlet. However, if you must use an extension cord, use a heavy-duty one. Never use the same kind you might use for a table lamp. Those are just not able to handle the wattage space heaters require. The new portable heaters have some very practical features. For instance, the Holmes Tower Unit has ceramic technology and a blower. It's adjustable and it turns to distribute the heat evenly throughout the room. You can even lay it on its side to use as a baseboard heater.Many manufacturers include automatic turn-off if the unit tips over. Some have features that sense excess heat in front of it, so if it's too close to a drape, for example, it will turn itself off before a fire can start.
Bionaire makes a model with a removable thermostat that you place at the other end of the room. It maintains the right temperature throughout the entire space. Some have remote controls with a nightlight feature.Parabolic models heat only what they "see" instead of heating the air in the room. You point the heater at the chair you'll be sitting in; you'll feel the heat, but it doesn't heat the air around it.
Oil-filled units are probably one of the most efficient units because they actually mimic the idea of a radiator. An electric element heats the units. When they reach the set temperature, they shut down. But since the oil has residual heat, they continue to provide heat.
The Comfort Breeze Duct Booster isn't a heater, but it can increase the efficiency of your existing system. Slide one over the duct, and it will pull more heat out of the duct and into the room. Wire the duct booster so that it runs only when the HVAC system fan runs for maximum efficiency.