* Paul Dini was originally going to call Harley "Columbine" before deciding Harley sounded better.
* The names of Joker and Harley's two pet hyenas are Bud and Lou - a fact I knew, that has only recently been revealed in the episode "Joker's Millions" (in case anyone din't guess, that's in honor of Bud Abbot and Lou Costello)
* Harleys' real name "Harleen" is just her voice performer's name "Arleen" with an 'H'!! Obvious I know, but cute!!!
* Not strictly Harley: She has not been the only gal after Joker in all of time!! A character called "The Mime" came before here ages ago, and wuz Joker's first girlfriend!!!
*She also not the first female character in DC to bear the name Harley Quinn. Back in the seventies, there wuz a member of the Teen Titans and also called "The Daughter of the Joker" which she wuz from where she came. However, she wuz a goody!!!boooooo
* In the episode "Harlequinnade" Harley says the line "I'm not even a real blonde". Despite this, she is in fact, just that!!
* In Finland "Mad Love" is called "Mielipuoli Rakkuus"
* The song Harley sings in the episode "Harlequinnade", contrary to popular belief, was NOT written by Paul Dini! It is actually a song called "Say that We're Sweethearts Again" from an old MGM movie called "Meet the People".
*Just for fun, here's what Harley's name means....
Harleen/Harley: from the long field.
Quinzel/Quinn: Fair hair or a noticeably pale complexion
You can figure how that relates to Harley yaselves!!=-)
* The name "Arleen" means "the pledge". It's original form is "Arlena" and it is Hebrew. It became "Arleen" when the Celts and the Gauls began to use it.
*Not strictly Harley: Back in the 1300's there wuz an Irish clan called "The Napier Clan"!!! (Jack Napier is rumored to be Joker's real name).So when Joker says in the episode "The Laffing Fish" 'I'm Irish' he ain't kiddin!!;D
*Harley's not the first to bear her synonym!!Fans of Agatha Christy will be familiar with the story "The Mysterious Mr.Quin", centering around the supernatural, mysterious gentleman Harley Quin, whose face is often cast in shadows, seeming like a mask, and whose checked suit often has light thrown upon it, turning it into a motley of diamond patterns!!
*Not strictly Harley The Italian word for Harlequin is "Arrlechino".When the character Harlequin ran away with the character Columbine, they moved to France and changed their names to "Peirot" and "Pierette".
*In the episode "Harley's Holiday" the idea of Harley accidentally leaving the security tag on her new dress wuz thought up by Arleen Sorkin!! The character she played in an episode of Perry Mason wuz a thief,but her idea that it would be funny if the character left security tags on her clothes wuz turned down by her director!
*Not strictly Harley:The character Harlequin may have been derived from the Norse Underworld Goddess "Hel" Her consorts wore a mask called a 'helkappe' which rendered them invisible!
*The Fox Network wanted to make damn sure that The Joker didn't come off as gay, especially considering that Tim Curry (Rocky Horror) was the original choice to play him, so they wanted a girlfriend created for him.This wuz the original reason behind Harley's conception!!!!The Joker???Gay???I don't think so!!!! He's BI!! Nah, actually,he's just too insane to care!!
* Look out for the new Creeper episode "Beware the Creeper" .Joker and Harley guest star. I can't say any more at this point,but as soon as it airs, I'll put up the exciting tidbit!!!Well, it's aired so I can say!!! The line is one of the most raunchiest,sexually oriented lines to go to air uncensored in a kids cartoon, which is why it's so special hehe.Harley jumps out of a giant pie, scoops some of it on her finger, beckons to Mistah J and says....."Wanna try my pie, Puddin?" LOL!
*Harley is DEFINETELY bi-seuxal folks, and in a semi-relationship with Poison Ivy!!! Oohhh yeah!
* Harley's middle name is "Francis". This is also the middle name of Arleen Sorkin!
*Not strictly Harley: Since Joker's creation that fateful Spring of 1940, in Batman #1, DC Comics writers have devised Fifty Four different nicknames for him!!!! They're listed alphabetically below:
1.Ace of Knaves
2.the Arch Buffoon of Banditry
3.the Bandit Buffoon
4.the Brazen Buffoon
5.the Brazen Bufoon of Crime
6.the Burglar Buffoon
7.the Caliph of Clowns
8.the Clown Prince of Crime
9.the Clown of Crime
10.the Comic of Crime
11.the Crafty Clown of Crime
12.the Crime Clown
13.the Crime Jester
14.the Cunning Crime King
15.the Cunning Caliph of Crime
16.the Fearsome Fakir
17.the Fiendish Funster
18.the Funny Man of Felony
19.the Grim Jester
20.the Grinning Gargoyle of Greed
21.the Harlequin of Crime
22.the Harlequin of Hate
23.the Hilarious Harlequin
24.the Hoodlum Harlequin
25.the Jaunty Jester 
26.the Jeering Jackanapes
27.the Jeering Jester of Crime
28.the Jester of Crime
29.the Jocund Jack of All Crimes
30.the King of Jesters
31.the Leonardo of the Larcenous Laugh
32.the Macbre Master of Mirth
33.the Mad Buffoon
34.the Mad Maestro of Mirth
35.the Madman of Mirth
36.the Maestro of Malevolent Mirth
37.the Mad Master of Mirth
38.the Mad Manager of Menace
39.the Mad Merrymaker
40.the Merry Madcap of Mayhem
41.the Mirthful Mountebank of Mischief and Menace
42.the Mocking Mountebank of Menace
43.the Mogul of Mountebanks
44.the Malevolent Mime
45.the Malicious Mountebank
46.the Man of a Thousand False Fronts
47.the Master of Menace
49.the Mirthful Mountebank
50.the Mocking Mountebank
51.the Prince of Pranksters
52.the Ringmaster of Riotous Robbery
53.the Spinner of Sinister Skeins
54.the Tycoon of Teasing Terror

If you know something you don't see here, let me know!!