Catholic Encyclopedia: St. Valentine

Three saints named Valentine are mentioned in early martyrologies. The third was an African martyr of whom we know almost nothing. The first two, St. Valentine of Rome and St. Valentine of Terni, are probably identical. One is called a priest, the other a
http://www.newadvent.org/cathen/15254a.htm

Valentine

Illustrated profile, with links.
http://www.catholic-forum.com/saints/saintv06.htm

Sacred Heart Parish: St. Valentine

Short, entertaining essay on the third-century bishop of Terni and martyr, and his feast day.
http://www.hullp.demon.co.uk/SacredHeart/saint/st_valentine.htm

Origins of St. Valentine's Day

AmericanCatholic.org article relating the origins and evolution of St. Valentine's Day, along with a short explanation of why the Catholic Church no longer officially celebrates the day.
http://www.americancatholic.org/Features/ValentinesDay/

For All the Saints

Hagiography of St. Valentine of Terni, followed by the story of St. Valentine of Rome. As you will see, they are suspiciously similar.
http://www.saintpatrickdc.org/ss/0214.htm#valt