Paint. By far the easiest thing to do to spruce up your home and the best bang for your buck. I was in a home this weekend and every wall corner had been scraped clean of paint. I guess kids dragged their hands around the corners or something, but this could have been fixed with a bit of paint.
I have seen homes with 5 or more different paint schemes. Black bedrooms, purple dining rooms, and many, badly painted wall murals. These all need to go when getting ready to sell your home. We use the word neutralize when talking about painting your home a neutral color. One of the most popular colors right now in our area is a light grey color.
If you are thinking about painting, don't forget the baseboards. A light coat of white paint on the baseboards really makes them shine!
The outside of your home may need paint as well. Check around all the windows and see if they could use a coat of paint. The front door needs to be looked as well. It usually gets beat up by the elements and probably could use a fresh coat of paint or refinishing.
Paint. The cheapest way to really make your home stand out!