Ok I will admit it, I will get up early just to see sponge bob ant the fairly odd parents. I am a grown woman- so my family thinks I am nuts. I know there has to be other adults that watch cartoons regularly- PLease share with me.
He-Man
Various Anime.
Jackie Chan Adventures
Justice League.
Oh yeah-- Thunderbirds.
In Supermarionation! 
The continuity-wrecking Spiderman and his Amazing Friends, Spongebob, and both generations of TMNT. Yes, I am sad.
Powerpuff Girls!!!
Sheeeeesh. Only one in this whole thread so far I’d call worthy of guilt is Spiderman and his Amazing friends.
Thunderbirds isn’t my favourite, but it’s not bad. Same with old-school TMNT. (Haven’t managed to catch the new one.) The rest that have been listed are brilliant.
Anyway…GUILTY pleasures…
Medabots and Yu-Gi-Oh.
Samurai Jack
Power Puff Girls
Ed, Ed, and Eddie
Dexter’s Laboratory
Guh? Just one? Nope, can’t do it.
Futurama
Simpsons
Family Guy
Robotech
All the Disneys (including the bad ones like Hunchback. I like em all),
Voltron (just the lions, not the crappy car series)
Sponge bob
The Incredible Hulk (circa 1966)
Undergrads
Oblongs
Quads
I love the Oblongs, and I wish that it got picked up for more than one season. Same with Undergrads…
Yu-Gi-Oh is pretty decent, sort of a more sophisticated Pokemon. I have never made it through an entire Medabots. Spiderman and those two x-men flunkies just amuse me to no end for some strange reason.
Dexter (and Power Puff here and there if I get sucked into an epidode)
Ren and Stimpy
There are a lot of animated shows that I’m just not embarassed to talk about. But beginning several years ago Cartoon Netword began making their own shows and they were great. But people tended to look at you funny if animation was on a network or someother adult channel at adult times. But I loved them.
Old Dexter was brilliant. Indeed I’d say some of these shows should rightfully be recognized as the best animated shorts ever.
The Fairly Oddparents is hilarious. The fastest pacingest show I’ve ever seen.
Old Rugrats was really fun. So many great adventures without ever leaving the yard.
Ed, Edd and Eddie is mesmerizing. I can’t really explain it. Their Plank short parodying some 70s kids show like Sesame Street is just so funny.
Space Ghost Coast to Coast was just … just … Hmm, words fail this show. Like the shrub episode. Or Zorak’s nephew. Or Tad’s brine shrimp.
Thankfully after so many years of great programming I no longer have to blush about loving the channel.
But the one that’s most embarassing are the Pretty Sammy episodes off of the Mishoshi Special DVD. You know the guy who makes Tenchi Muyo. Well he did this outrageous parody of Sailor Moon. So funny, so naughty.
The Wacky Races
Time Squad
Simpsons
Southpark
Futurama
Cowboy Bebop
Gary & Mike (does claymation count?)
Samuri Jack
Undergrads
I guess that’s about it
I don’t consider Spongebob a guilty pleasure…it’s a great show! Ditto the Simpsons, Invader Zim, Spaceghost, Looney Tunes, Peanuts, Rocky & Bullwinkle or Futurama. King of the Hill, Daria and the Family Guy have their moments, too.
Now, I’m slightly less proud to say I enjoy Beavis and Butthead. And the Smurfs. And the Flintstones, Scooby Doo and a host of other poorly animated, painfully scripted Hanna-Barbera shows. Or to admit having great nostalgia for long-lost shows of my youth like Rubik’s Cube, Dungeons and Dragons, and all the video-game based cartoons that cropped up in the wake of Pac Man fever.
Truth be told, I can only say that I’m occasionally, ever so slightly embarrassed about being a nearly 30-year old woman and still a big fan of cartoons (it sounds a little better when you call it Animation, tho). But I wouldn’t say that I feel guilty.
Dexter’s Laboratory
Powerpuff Girls
Justice League
Beast Wars (one of my favorite shows ever…own the series on tape)
anything on adult swim
I LOVE the Smurfs. I have a my Direct TV search on for showtimes all the time. haha
I also think Beavis and Butthead are hilarious. I really like Freakazoid too, but its always on in the middle of the night or something.
Oh yeah, I really the The Gummy Bears too, and also The Jetson’s. Okay, and the Flinstones.
I love South Park (but try to keep that to myself around my co-workers). I miss Daria. I REALLY miss The Tick 
I’m sorry if he offends any French, but Pepe LePew has always been my rolemodel for wooing women.
http://webpdp.gator.com/v3/webpdp_v3_plugin.php?yic=HIC_Infospace1
Honorable mention to Michigan J. Frog, the batrachian Fred Astaire.
correct LEPEW LINK: http://rturnoffly.freeyellow.com/PepeLePew.html