Soul Eater

Soul Eater is a shonen manga by Atsushi Ookubo which has been regularly serialized in Monthly Shonen Gangan since 2004. It received a 51 episode anime by Studio Bones in 2008. As of January 2011, Monthly Shonen Gangan has ran a Prequel/Spinoff named Soul Eater Not! by the same mangaka, running alongside the original series while covering and expanding on events prior to the original story.
In the world of Soul Eater, a human soul that falls to corruption has a chance of transforming into a "Demon Egg", enough of which could create the ultimate avatar of madness.
To stop this from happening, Death himself has set up the Death Weapon Meister Academy (aka DWMA*) in Death City, Nevada. The DWMA trains warriors ("Meisters") and Weapons (humans who can shapeshift into weapons) to fight and retrieve the evil souls before they become Demon Eggs. A Meister and Weapon who manage to collect 99 corrupt souls and one Witch soul receive the highest honour: the Weapon becomes one of the Death Scythes, servitors of Death himself.
The main characters include:
Maka Albarn and her scythe, Soul Eater (just call him Soul). She's a raging workaholic, he's a slacker, but they make a great team. They almost made Soul into a Death Scythe at the beginning of the series, but failed on a technicality and have to start over.
Black☆Star and his Weapon Tsubaki: Black☆Star is a would-be master assassin who has trouble reconciling the concepts of "stealth" and "shouting about how awesome you are". His partner Tsubaki is an incredibly patient and capable Weapon who can transform into multiple ninja weapons, from a chain-scythe to a smokebomb.
Death the Kid and his twin-pistols the Thompson Sisters: Son of Death himself, whose incredible natural power is offset by his extreme OCD, being absolutely obsessed with symmetry. The Thompson Sisters serve as his weapons: Liz is the older and more cynical sister, while Patty is younger and more goofy.