Chrno Crusade

Chrno Crusade!
AKA, Nuns With Guns.
If you look at the image to the left, you'll see a girl holding a particularly large gun. Yes, that is a NUN. I kid you not.
The series is spectacularly done, with incredible graphics, fantastic plot, and rich characters that are as far away from flat as Mount Everest.
It goes by several names, depending on where you watch it. I highly recommend watching it in the original japanese, as the english translation is so awful that I turned my computer off and left the room. But if you watch it in english, it's Chrono Crusade. The 'Chrono', as it 'time,' is completely coincidental. Though our fabulous demon-boy Chrno/Chrono does have limited time-freezing power stored in his lost horns (which is how a demon ended up at a monestary as an excorsist FIGHTING other demons), it means nothing of the sort in Japanese, and other characters with time-powers do not have names that corolate with those powers. In Japanese, it's Chrno Crusade, without the first 'o', or if you get very literal in the translations, 'Chrno's Crusades.'
This outstanding series is about Rossette Christopher, an Excorcist working at the Magdalene Monestary, trying to find her brother Joshua who was taken by a demon four years previous. Her and Chrno (who is also friends with Joshua.) are trying to find him on their demon-killing escapades, and having little luck. Chrno is a mild-mannered demon who's lost his horns, without which, he would normally die. But, against his will, he made a contract with his good friend Rossette when she was still just a girl, that took her lifeforce rather than a Demon's natural energy source: astral. Every moment he lives he takes from her, though she does not regret it. Chrno, on the other hand, does.
I, personally, was hooked on this anime within the very first five minutes. It's a tad bit religious, but my Atheistic friend was still able to enjoy it. So long as you don't get super-into it, you should be good. I give Chrno Crusade Five Stars.