It's a relatively new manga, but I quite like it. Wondering if anyone else does too. ^^
Fairly original storyline (bit similar to D.Gray-man though) and, well, yay for anti-heroes!

Summary
This world consists of two dimensions joined as one, like a mirror. The first is the world in which the humans live, Assiah. The other is the world of demons, Gehenna. Ordinarily, travel between the two, and indeed any kind of contact between the two, is impossible. However the demons can pass over into this world by possessing anything that exists within it. Satan is the god of demons, but there's one thing that he doesn't have and that's a container in the human world that is powerful enough to hold him!! For that purpose, he created Rin, his son from a human woman, but will his son agree to his plans, or will he become something else...? An exorcist?

Links to download;
http://mangahelpers.com/m/ao-no-exorcist/scanlations/
There's only been three chapters released so far though, as well as RAWS for the fourth.