Hellsing Ultimate and TTGL were already mentioned.
Then there's Berserk - Medieval fantasy about the leader if a mercenary band aiming to become a king with the help of his right hand.
Vision of Escaflowne - Fantasy with steampunk mecha (better than it sounds). A girl is transported to a different world, where an empire is apparently looking for Atlantis.
Redline - Awesome racing IN SPACE!
Ano hi mita hana don'tremembertherest - A group of friends drift apart after one of them dies in an accident. Years later, her ghost appears before her closest friend, looking to get into the afterlife. Expect lots of bawling and an overexaggerated ending.
Then there's good ol' Cowboy Bebop - Cowboy headhunters IN SPACE!
The Melancholy of Haruhi Suzumiya - This one defies classification. It's a story about a girl who announces that she doesn't care about ordinary people, just ESPers, aliens or time travelers. Next day, she meets them.
Black Lagoon - Mercenaries being mercenaries.
Armitage III - Oldschool Cyberpunk.
Afro Samurai - Samuel L Jackson. That is all.
Also, shame on you. Darker than Black is awesome.