Okay, I have read 3 of the 9 volumes of Preacher and I am absolutely in love with it. Any series you can think of that are similar in terms of story and art, or anything good by Garth Ennis and Matt Dillon?
I also am interested in 100 Bullets.

Punisher MAX series.
But I'm also a huge Punisher fan, so I might be a bit biased.
EDIT: Also, I should probably mention that the Punisher MAX series is VERY graphic, but also features the most realistic depiction of Castle, and his methods of killing/torture get incredibly....inventive.
Ennis does do the storyline, and is the best writer to ever work on ol' Frank, but I don't believe Dillon does anything with the MAX story arcs. I believe Dillon did work on Welcome Back Frank and others after it tho.

The only way in which I find 100 Bullets and Preacher similar is that they are both good,
Besides, aren't you too young to be reading this shit?

Transmetropolitan, perhaps?

I'm just fucking with you, I say focus on reading 100 Bullets and then try to find other shit. 100 Bullets is pretty fantastic, take my word on it.

Excellent stuff. Love that. I seem to have misplaced about half of my collection so I'm looking for the comics in the stores again. Which I can't seem to find anywhere.
And mine.

Well... the graphic novel format is very young, so there isn't exactly a huge backlog to recommend. If you're looking for more "10 volumes of good", then there's always Sandman, Y: The Last Man, the Alan Moore series (Tom Strong, Promethea, The League of Extraordinary Gentlemen, Top 10... well, actually, those are only three to six volumes), and Sin City, I guess.
I also find Lone Wolf and Cub to be an excellent read (if somewhat reptitive - I honestly can't recall what the differences between volumes are), but I'm having trouble expanding my collection as they're mostly out of print. Specifically, I can't find volumes 12 and 13 anywhere, meaning I'm stuck for now. I should mention that they're printed western style. I have no idea how they made that work.
And, of course, if you don't mind "limited" series, there's always the Alan Moore ""top three": From Hell (very graphic, both in gore and sex), V For Vendetta, and of course his absolute masterpiece, Watchmen. Awesome.
Frank Miller also has some good non-Sin City stuff, namely Ronin, The Dark Knight Returns and, of course, the one and only Batman: Year One. The Dark Knight Strikes Again? What's that?
(Should mention I almost spleled "Watchmen" as "Watchmane"... Huh.)

Hellblazer. I think, that Ennis was kicked from it, when he start to be... too Ennis,

Hellblazer
Lucifer

I fell in love with Preacher when it made me laugh out loud.
A guy turns to another who has just had his lower jaw blown away and asks 'You think that was supposed to be a hit?' The expression on the other guys face is just perfect.

As Wraith said, Punisher MAX. I can safely say it's not just for Punisher fans--I didn't even particularly care about the character before starting in on it, but it's every bit as sharply written and blackly funny as Preacher.
100 Bullets is something completely different, but still great.

@ Honorbuddy: I suppose you want me to read the Sunday comics instead then? =)
I love the Punisher series, didn't know there were ones done by Ennis.
I've studied up on other Vertigo comic series while in detention (In the computer room oddly) so I'm pretty sure I know what they are.
Thanks

I liked punisher max, but the "classic" ones (ie not max) get boring really fast. They are indeed about as bleak as preacher.

Well, if you can stomach Marvel superheroes, I'd highly recommend the Guardian Devil series from Daredevil. It's only about eight issues I think, I have the whole story in two hardcover books but I enjoyed it a lot and the theme is somewhat similar (Antichrist, devils etc.).
Sure, the story loses it's momentum towards the end and the finale is somewhat disappointing but I'd still recommend it. The art is just lovely (as is the Black Widow ;) ).
Example of a cover: http://www.panelsonpages.com/wp-content/uploads/2009/07/dd-cover.jpg