And yes, that's three seperate links.
As near as I can tell, he was originally the creation for a comic series named Hellblazer. He was quickly coopted and given cameos in many many MANY other comics, ranging from Sandman to Swamp Thing to stuff I never heard of (not that that's hard exactly).
The Lady Joanna Constantine is not original to Hellblazer, but is rather an artifact of Sandman and a series called "Dreaming" that I theorize is related but cannot confirm. Other things to look out for if you happen to be a Sandman addict (hi claire!) - Sandman #13, one of the "long time ago" episodes, look for a Jack Constantine. He shows up again in Dreaming. John Constantine shows up in the Books of Magic, as you well know if you've read them, as does Thomas Constantine, John's father, and Cheryl (Constantine) Masters, John's older sister.
And in the Swamp Thing, there's an earth elemental that's conceived using John's body. Hee hee. How cute ^^
current mood: I play computer games and listen to Marilyn Manson and therefore have no regard for my life.
(well, no. not really.)