Daphne from Scooby-Doo is the earliest crush I can remember – sometime in my late single-digit years. She gave me the jinkies.
Me, too. Although Daria presents a bit of a problem; my friends say I’m the love child of Daria & Ross Geller from Friends, so things could get a bit squicky.
I had a crush on Inspector Gadget’s Penny, though I saw the show almost never.
At various other times…
Kim Possible
Erin Esurance
Ariel
Belle
Giselle
Jasmine
Hello Nurse
And even though she’s a stick figure, Haley from OotS is pretty hot.
Sister Death in Neil Gaiman’s Sandman comic books.
(I distracted myself and held up my fellow party-goers at a bar we were leaving from for twenty minutes, because a perfectly petite, gorgeous, pale-as-snow Death walked in wearing a top-hat, a black coat, lots of belts and little else, this halloween. My down-to-earth friend snapped me out of it by levelly asking her where her character was from. He knew very well, but she couldn’t answer - eventually it turned out she’d just seen the picture on her friend’s tattoo and fancied it. Curses! Or I’d probably still be there. Still, checking out Death as Alex DeLarge was quite amusing. And damnit, she was hot!)
Brock Samson. Batman from Batman: The Animated Series and Justice League. I guess I like my alpha males to be cartoonishly alpha. 
Have you seen the parody in the Spaceballs series?
Rick Hunter and Zor from Robotech. If comics count, Kyle Rayner’s Green Lantern and Jaime Reyes’ Blue Beetle.
These characters are from fairly obscure cartoons from the 1960s:
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Tarra from the “Herculoids” cartoon
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Sheera from the “Mightor” cartoon
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Jan from the “Space Ghost” cartoon
Even as a prepubescent, I was attracted to the animated representation of the female form (especially when clad in tight, colorful, and/or skimpy costumes!), as evidenced by the three heroines I mentioned above. Does any one else remember those characters or the cartoons that they starred in?
My “superheroine-in-peril” fetish/paraphilia started early – My interest in the characters kicked into high gear when I saw the heroines get KOed and/or captured by the bad guys!
Also, the more widely-known characters – Daphne from Scooby Doo, Susan Storm/Invisible Girl from the Fantastic Four, Batgirl, and of course Wonder Woman in Super Friends (it did not debut until 1972 or 1973).
Ah…cite? 
Maid Marian? Yeah, she’s a fox.
Batman from the animated series. sigh.
Currently? Tip Wilkins, from Skin Horse. Oh, Tip. How can you look that good in a dress, and still be that relentlessly heterosexual?
Betty Rubble (and no, I would not be thinking of Wilma).
Ooh, good call. She had slipped my mind but now that you mention it she might just have been my earliest crush too. ZOINKS!
Have to go with Princess from Battle of the Planets.
The female crew member of the Voyager from Fantastic Voyage. I don’t remember her name but she was smart as a whip as well.
For me, it’s all about the voice. Certain voice actresses have just the right sultry, scratchy timbre that short-circuits my X-factor gene.
Penny, from Inspector Gadget
Jane, from Daria
Raven, from Teen Titans
Erin Esurance
Bastila, from Knights of the Old Republic
Hmm…let’s see.
When I was a kid I had a mad crush on Speed Racer.
Kimba the White Lion
Deslok of Gamelon (can’t find a pic–an old anime villain from the 70s).
Do video game characters with cinematics count? If so, toss in Sephiroth, and Ghaleon from “Lunar: The Silver Star”.
Magneto from Marvel Comics. Ohhhh, yeah.
And I have a bit of a girl-crush on Kim Possible and the Esurance girl.
Oh, I forgot one–I really had a crush on Max from “A Goofy Movie.”