Well, since you said it first, I will admit that I’d sleep with Professor Lupin in a heartbeat. (However, I can’t shake the sense that I am a pervert for feeling that way about a character in a children’s book, even an adult character.)
I’d take a few that have been mentioned already (Stu Redman, Aragorn, Professor Snape), but I’ll add a few new ones.
Han Solo: I’ve had a crush on him since 1980
Almost anyone from Buffy or Angel: Sunnydale, the town where even reject nerds are extremely attractive
Harry Dresden from Jim Butcher’s Dresden Files series: it would probably mean terrible things would happen to me, but I don’t really care. 
G’Kar from Babylon 5: G’Quan help me, I’m into the lizard dude. In my defense, it’s all about his personality. The rest of the Narns don’t do it for me. I’d also go for Captain Sheridan, whose personality I also enjoy, but I’ve dug him since he and Kate Jackson were secret agents…
Marvels Comics’ The Mighty Thor, especially the current version with the beard. Yum.
Batman - but only if he wears the cape and cowl.
Ferris Bueller
That’s an eclectic list. Am I really, really weird?
Janet & Chrissy from Threes Company
The Lady Jessica Atredies. Oh my GOD yes.
And even Princess Irulan from the SCI-FI t.v. version. That dinner scene rules - no where near as good as the one in the book, but mmmm… Irulan…
I’m amused by the multiple votes for Mr. Darcy, with no mention for Jane Austen’s female talent. I don’t know about anyone else, but I would personally sleep with Elizabeth Bennett anytime, anywhere.
For another classic literary crush: Beatrice, from Much Ado. Who could resist? Kate from Shrew is also very attractive, at least until the last scene; but in this case, I think the re-make is even better, so Kat from 10 Things I Hate About You comes in third place.
Just to show that I didn’t spend all my time on lofty works… I always had a dreadful infatuation with Beverly Crusher, from ST:TNG.
Captain Jack Sparrow.
Philip Marlowe.
Well mostly Anime derived:
Mahoro from Mahoromatic Maid - cute, nice, love the goofy hair, and I won’t have to worry about a long term relationship. :eek: :eek: 
:o 
Most of the female cast from from Tenchi Muyo! except Sasami and Washuu. I am not sick or crazy.
The Sailor Senshi. All of them. At the same time. (Except Chibi-Usa)
Faye Valentine from Cowboy Bebop. Those hotpant - wow.
Bellandy from Ah! My Goddess - nice, kind and generally plesant person who is also very attractive.
Kodachi from Ranma 1/2 - my kind of Crazy girl.(Though I would have to wear a Ranma Mask.) Also Kasumi, for the same reasons as Belldandy.
Excel from Excel Saga - bet she would be really active in bed, and those eyes and hair are amazing. Hyatt wouldn’t be bad, but the sex might kill her, and while she would get better, sleeping with dead people is pretty digusting.
Lina Inverse and Naga the White Serpent from Slayers would be an interesting duo, but Lina would probably Dragon slave me for even asking.
Kazami Mizuho from Onegai Teacher, well, makes me hot for teacher indeed.
And finally, any of the female extras from a random hentai tentacle anime. 
Jessica Rabbit.
Trillian (Hitchhiker’s Guide series)
Mary Anne AND Ginger, simultaneously. (Sleep with, right? Not maintain a conversation afterwards).
If I were only 12 inches tall, the twins that hung out with Mothra. Man, I had a thing for them when I was about 8 years old.
I’d switch teams for Dr. Frank N. Furter.
Shouldn’t that be
Batman - but only if he is prepared?
QuizCustodet will have to get out of the way: not only would I sleep with Lizzy Bennett, but I’d marry her, too.
Come to think of it, the women in fiction I’d be interested in, I would rather go for exploring a long-term relationship, not just for one time: like Franny Glass from Salinger’s Franny and Zooey. Difficult, true, but worth a try.
A fictional character just for sleeping with once: Lara Croft. I’ve seen the last movie, but actually I’d be more interested in the way she looks in the games.
Obi-Wan Kenobi (Because Ewan McGregor is so fucking HOT!)
Ditto Dr. Luka Kovac and Dr. John Carter from ER. Mmmm…Luka…
Darkwind K’Sheyna from Mercedes Lackey’s Valdemar series. I’d add Vanyel, but he’s gay, unfortunately, so it wouldn’t work out.
Lloyd Dobbler, without a milisecond of hestitation
Rupert Giles from Buffy
Mr. Rochester(I’ve mostly gotten over this, but in 8th grade, this was a major crush)
Sirius Black from Harry Potter(I like Lupin too, but it’s all really Sirius for me)
-Lil
Montag from farenheight 451
Mr. Darcy- Pride and Prejudice
Inman-Cold mountain
Lloyd Dobler
Roland DeSchain(sp?)
Neo (so long as he’s not faking it… cuz the boy can’t act)
Crash Override from Hackers
Christian from Moulin Rouge
Sick Boy from Trainspotting (w/ a condom)
Albert Packer from Cannibal: the Musical
Wesley from Princess Bride
And last but CERTAINLY not least, Brian and Michael from Queer as Folk. Unf unf.
Berg (from 2 guys and a girl)
Fox Mulder
Legolas
Edward (from the Anita Blake books, who doesn’t want a piece of Death?)
And on the BtVS front, while Spike has sightly more appeal than Xander, I think I’d have to go with Xander for even the most casual of flings. A. he can be outdoors during the day b. he’s not the stalker/ weepy psychotic type.
Little Mermaid w/legs(preferably)
Jessica Rabit…
Stripparella
and Samantha Jones from Sex and the City…
Damn, absolutely seconded.
Lazarus Long from the later Heinlein books. There’s a man who really knows how to appreciate a woman.
Taylor from the original “Planet of the Apes” film
Captain James T. Kirk
Illya Kuryakin
Rob Gordon (John Cusak) in “High Fidelity”
Hagrid. You know what they say about men with big feet…
I will second Elizabeth Bennett.
Definitely Zatanna, from DC comics. John Constantine has good taste.
And Jean Harrington (Barbara Stanwyck’s character) in The Lady Eve.
Off the top of my head…
Malcom MacDowell as Alex in “A Clockwork Orange”
John Cusack as Rob Gordon in “High Fidelity”
…and maybe Julia from “1984”