My roommate and I have noticed that, since our apartment faces south and we live right by the Hudson, our apartment is starting to be quite chilly in the evenings, and oftentimes is cooler inside than out. We haven't started turning on our heaters yet, and yesterday she suggested we buy space heaters as a solution for keeping our (fairly empty) apartment warm enough to read or surf the internet in without having to bundle up indoors or have our window heating units running for extended periods of time. (This gets expensive, and we were able to cut our electric bill by 75% by running the AC for short periods in the evenings alone last month.) Naturally, we would unplug and put away said units before we left for work each morning.
Our current built-in units are Islandaire brand under-window units, EZ Series, 42. We haven't picked a space heater yet, but chances are I'd just pick one up from Target.