This theme song got me as a kid.  I couldn’t stand the cartoon itself, but I couldn’t get enough of the intro.  C’mon, it’s inspirational!

a simple shoeshine boy... or is he?

When criminals in this world appear,
And break the laws that they should fear,
And frighten all who see or hear,
The cry goes up both far and near for…
Underdog! (Underdog!)
Underdog! (Underdog!)
Speed of lightning,
Roar of thunder,
Fighting all who rob or plunder,
Underdog (Underdog) Underdog!

Here’s the intro itself.

And Scrubs did it a cappella (starting at 1:40).