Gunslinger Girl is an ongoing manga by Yu Aida. It was adapted into a 13-episode anime television series (produced by Madhouse and Bandai Visual) which aired in Japan from October 8, 2003 to February 19, 2004.
This list is somewhat incomplete due to sidereel not having certain series in their databases yet, see the full updated list here http://myanimelist.net/animelist/raar