It's been ages since I read a story that I wanted to recommend - mostly because I don't read a lot of fanfic these days. This one made me laugh several times and I really thought the author had the voices down pat.
TITLE:
Where There's SmokeAUTHOR: TozaBoma
FANDOM: Constantine (Hellblazer + TV)/Supernatural
CHARACTERS: Dean, Sam, John Constanine
GENRE: Genfic - Action and Humor
RATING: Teens and up
SUMMARY: Sam and Dean find an unlikely ally in the war against… library books. John Constantine? All he wants is a quiet life. That's not the canon way though, right? Rated Teen and Up for adult themes, slightly horrific things that go om-nom-nom in the dark, sinister tones, a bit of rumpy-pumpy, and John's swearing.
NOTES: This is set after Supernatural 10x04, so contains references and spoilers. Constantine is a mix of Hellblazer!John and MattRyan!John.
I think one of the reasons why I like John Constantine is that he sounds just a little bit like Spike. It makes me all nostalgic to read beloved expressions like "tosser". Anyway, this is a thoroughly entertaining story, with a lot of plot. The crossover works so well, it seems like the two shows were made for each other. :)