If this spring's Fate/zero blood drives demonstrated anything, it's that otaku will line up around the corner to give blood in return for a character poster. The idea is being put into play again at doujinshi mega event Comiket 82 with posters given to attendees who donate 400ml of blood.
Posters feature characters from the Cardfight Vanguard, Broccoli, game makers (often adult ones) TYPE-MOON, Leaf, Nitroplus, Unison Shift, Minato Soft, Cotton SoftNekoneko Soft, and Nitroplus. However, perhaps the best is the titular anti-hero of Mahjong Legend Akagi: The Genius Who Descended, featured with adversary Iwao Washizu - debauched, uber-rich, uber-powerful gambler who plays mahjong for blood.
A similar promotion was run at Comiket 81.