Well I can only remember a few were I got the case of the giggles:

Quote
Clark's mother trying to teach him to fly, and then pushing him off the balcony.
Me too.

I laughed when Martha said something like, "well they won't be looking at your face." The part when she made Clark's Superman outfit.

I'm sure there is more . . .

Though, I laughed at the show a lot in season 4 and during IOM. AH TOMATOES.

EDIT:

I remembered another one:
Quote
"Lois is…well, she’s…complicated. Domineering, uncompromising, pig-headed…brilliant. And we’re not really going out. It’s business.” (Clark)

“Uh, huh.” (Martha Kent)
Thanks to LoisLane9397! Martha and Perry usually got me to giggle.

Lex did too - "I think I'm going to throw up." When he was skeet shooting off the top floor of his building and he said something along the lines of, "call the lawyers Nigel."

I've converted to lurk-ism... hopefully only temporary.