Razor (2007) tells the story of things on the Pegasus. It also stars an actress that was never in any of the TV episodes. The Plan (2009) explains some of the mysteries of Battlestar Galactica and is to be watched after you have seen the series finale.
http:http://en.wikipedia.org/wiki/List_of_Battlestar_Galactica_(2004_TV_series)_episodes//
http:http://en.wikipedia.org/wiki/List_of_Battlestar_Galactica_(2004_TV_series)_episodes//