This is probably the first technique one will think for creating soft shadows when doing shadow mapping.  The shadows are rendered to a texture (in screen space) and this texture is then blurred (in screen space) and later applied to the screen. This is a very easy technique for getting soft shadows. The main drawbacks are shadow bleeding and the cost of the extra passes.

An article about it on gamedev.net:
http://www.gamedev.net/reference/articles/article2193.asp