Yup, give
Gunslinger Girl (which often gets mistaken for a Noir-like show, but in truth it's totally different) a try for sure, fantastic series.
Also I highly recommend
Kousetsu Hyaku Monogatari, which is very dark, disturbing, with some great depth. The art-style is unique so it may turn off some people at first (though I love the art style right from the start), but just keep watching, and after a couple of episodes the plot and depth will surely grow on you. At least give it a try
Planet Es is also one of the best on-going series, it's more geared towards a mature audience. The story is awesome, it's based on a manga which has won the Japanese Nebula Award.
Full Metal Panic? Fumoffu is insanely fun, and a definitely must-watch if you love comedy. The humour and parody are just some of the best ever.
