-Black Lagoon
-Ga-Rei Zero
-Kara no Kyoukai
-Baccano!
-Darker than Black
-Death Note
-Higurashi
and
Mahou Shoujo Madoka Magica (trust me, it's not what it looks like it is. Give it a try, you won't regret it.)
if you're into darker / more serious anime, Deadman Wonderland is not bad.
A little gory, but the story plot has been pretty interesting so far.
It's about the prison for prisoners on the death row (hence the title), and the trials / games / competitions that they are put through in order to survive and delay their death sentence.
You can read the sypnosis here! - http://en.wikipedia.org/wiki/Deadman_Wonderland
A more lighthearted anime would be Ao no Exorcist, a typical shounen manga storyline with a child of the demon as its main character.
(Sypnosis here - http://en.wikipedia.org/wiki/Blue_Exorcist