Sheldon: Snaparoo!

Sheldon: Smurfingly smurftastic.

Admittedly, somewhat inspired by previous suggestions, but also somewhat improved.

"Sheldon: Where a kid can be a kid and so can his duck."

"Sheldon: An ordinary billionaire kid, his Gramps, his talking duck, their pug, and his ducks lizard son. Clearly, it's a family thing."

"Sheldon: No cats or girls allowed."

"Sheldon: Proving once and for all that your pets understand you, they just won't obey you."

"Sheldon: Worth having people think you are crazy when you remember it and laugh again later."

My vote goes to punxking's "All for fun and fun for all."

Yes, I read all the way through to the seventh page. Yes, that makes me crazy...

I think the main problem is that Sheldon is hard to boil down to a short tagline.

I think it's kind of grown organically from the beginning, which still wasn't exactly a one-tagline sort of concept: Billionaire boy who lives with Grampa who makes a talking duck. Dave's a twisted evil genius to make us try and think up one.
There's just too much awesomeness going on. And we all have our favorite characters/concepts, which might not be the highlight for someone else.

So in the spirit of that...

Sheldon: Too much awesome-sauce for one tagline.

Sheldon: It's all about the duck. Unless it's about the pug dog. Or the lizard. Or something else.

More suggestions! Yay!

Sheldon: Geek humor since 2001

Sheldon: A super-awesome comic by the super-awesome Dave Kellet

Sheldon: Your daily dose of awesome

Sheldon: Got Awesome?

Sheldon: Great art, great humor & great stories. Pick two.

Sheldon: It will make you cool

Yea I know it's long but...

Sheldon: The story of your average billionaire ten year old and his dog… and his talking duck… and his duck’s lizard son… who all live with the boy’s grandfather. Yup, nothing odd ‘bout that. Nope not at all…

and

Sheldon: Who says 42 flobbity–jillion dollars can’t buy you a talking duck (And his lizard son)

Okay one more

Sheldon: A duck above the rest

Sheldon: Just your average boy billionaire with a talking duck

Sheldon: It's like Tolstoy. But with a talking duck.

Sheldon: Because today's Peanuts is a repeat.

Sheldon: Guaranteed to make you more attractive. (Attractiveness not guaranteed.)

Sheldon: The webcomic of non-informative tag lines.

OK, one more.

Sheldon: Reducing workplace productivity since 2001.

Unusually appropriate at the moment...

Your Daily Dose of Duck

Sheldon: We put the "fun" in "dysfunctional"

(Note: I do NOT, personally, think of the Sheldon Family as dysfunctional - just lovably odd - but the term has come up a few times, so I thought I'd run with it.)

A few more. I'm bored and love free stuff.
Sheldon: Our Sauce is Awesome

Sheldon: 1 Duck was harmed in the making of this comic

Sheldon: Like those other comics, but funny!

Sheldon: Better than the Eighties!

Sheldon: For Geeks, the people who love them, and the people who love to hate them!

Sheldon: For Some Reason this comic reminds me of how much I liked eighties cartoon Jem

Sheldon: A bootful of cute.

Sheldon - Only comparable to Calvin & Hobbes!

Sheldon - Like the cheese on the macaroni!

Sheldon - The guy in Hong Kong likes it too!

Sheldon - Ain't it grand?

This Comic Will Quack You Up

Let's Get Quacking

Avoid Your Job For A Minute A Day

Sheldon: A comic strip for the average Joe

http://www.sheldoncomics.com/archive/051029.html

I've got ten favourites out of all of these! But my current top two are:

Sheldon: For anyone who's ever been a dork, a gramp, or a duck. by SatansParakeet

and:

Sheldon: when life hands you billions, create a talking duck by Crash (I've removed the word 'you' from this one. And I prefer it without adding the lemons, as someone else did.)

I think lee oades had it right: we need to avoid in-jokes, tags that are too generic and wrong impressions. Not easy!

My offering?
Sheldon: You'll believe a duck can talk

Read through posts and didn't see either of these. Apologies if I missed it.

Laughter is the best medicine. Ask your doctor is Sheldon is right for you.

A ray of sunshine even on cloudy days.

Kik-kik-kyah!! I love it! I love it! - Roscoe P Coltrane

This isn't really a tag line, but I wanted to try and make a full sentence using just the names/species of the characters. Here's are the best I could come up with:

Wealthy boy dogs his gramp, lizard ducks!

I know, lame...

I'm a fan of Rory's "A Duck Above the Rest" tagline...

with this contest I got to thinking, we're basicly summing up sheldon. If I really had to, I'd sum it up as,

Sheldon: rad.

Are ya feelin ducky?

Aw shucks, thanks Shrewd
Figured I'd throw 2 more out there whilst I was thinking of it.

Sheldon: If we told you then everyone would want in on it,
Sheldon: And you thought your family was multicultural

Sheldon: If it wasn't digital it would be cut out and on your wall.

Sheldon: Bringing back the good times

Sheldon: Imaginating the future of webcomics

Sheldon: Because you can't make toast underwater

Wait, I lost my train of thought ..

Sheldon: Whimsical, geeky fun!

Sheldon: More fun than you can quack at!

Sheldon: Smile at the ducky!

Sheldon: A duck, a pug, and a lizard can't be wrong.

Sheldon: Genius isn't always boring

Sheldon: Redefining the nuclear family

Sheldon: Fun to read! Find out how!