Looks like the post credit spoilers haven't been posted yet:
>First: Hellboy is crying over Broom's grave, Lobster Johnson's ghost (we met him alive in the flashback to HB"s origin) pops out and tells him he needs to get his shit together because HB won the battle but the war is coming. HB freaks out like a fanboy.
>Second: During the film HB makes a deal with Baba Yaga to exchange his eye for information, he leaves before she can collect. In the scene Baba Yaga is telling someone with an Eastern European accent that she'll let him "finally die" in exchange for HB's eye. He's not shown but it's obviously Koshchei.