Just a couple:

Sheldon - It's kind of like Richie Rich - but funny.

Sheldon - You will believe a lizard can fly.

In all honesty though - I think Dave came up with the perfect line himself a long time ago.

Sheldon - Pure ducky goodness.

Sheldon: The most duck for your buck!

Sheldon: Because newspapers are icky.

Sheldon: THIS is why the 'Net was invented.

Sheldon: Lowering employee productivity since 2001!

Sheldon: The best webbed feet on the Web!

Sheldon: Families come in all shapes and sizes... and species.

Sheldon: Picking up Scott Kurtz's readership overflow since 2001! ;P

Some more...

-A window into your childhood innocense.
-Read Sheldon and be one of the cool kids!
-It's like ex-lax for laughter!
-Sheldon- The comic strip that doesn't judge you.

Revision of someone else's:
"Awesome sauce, with a glass of Rad juice!"

Sheldon - You can't improve upon perfection!

Sheldon - Welcome to the Sheldom (you know, like kindom?)!

Or, Sheldon - Ripley's Belive it or Not has nothing on this strip (just for laughs).

Sheldon - 'Tis the meaning of life.

Sheldon - Describing it is like trying to describing life itself (oooo, deep)!

Sheldon - Don't...don't you want it? You know I don't believe it when you say that you don't read it! Go Human League!

Okay, okay, here's the last one.

Sheldon - SET PHASERS TO EXCITEMENT!

http://www.sheldoncomics.com/archive/030905.html

I realize that it's already a few hours over the deadline... but I just signed up and I can't contain myself. It's still Friday somewhere... right? Maybe?

Sheldon: The myriad adventures of a really rad kid.

And of course all of his family members/pets/pet's family members, but that would be silly to state in the tagline. They are, after all, aspects of Sheldon's life that just make him all the more rad himself for having them as a part of his life. :)