When it is cold outside, south-facing windows are awesome. They help keep the entire house snug and warm and let in tons of light. In fact, south-facing windows are best for daylighting. They let in the light all day long, and don't have as many problems with glare and heat as east- and west-facing windows.

But those lovely south-facing windows that keep a house so warm in the winter can also keep a house warm in the summer - even more so.  Unsurprisingly, this is appreciated this somewhat less.  

Don't dread the summer because of south-facing windows. With some simple tips it is easy to use them to full advantage. The Energy Saver's Windows page provides a lot of links with suggestions for managing windows to reduce the amount of heat from the sun (also known as solar gain) in the summer.  For instance, it helps to use window coverings such as blinds or curtains, install awnings or sun-control films, and plant deciduous trees in a location to cast shade on the window. Using indoor plans on the inside of the window will also provide some shade relief while giving the plants some much needed sunshine.