"Holy Crap" is always fun to say, and appropriate for almost any situation.
I also swear a LOT, even on facebook, which is why I try not to have too many family members as friends on there. And no, I won't just "not swear" because swearing is too much fun. Especially fuck. Fuck is a useful word.
Aside from profanity, while I don't really have a catchphrase, I am very "noisy" I guess. I make little grunts and such for just about everything I do. It's pretty confusing for people who don't know me well when I do it, but fuck them! I do it because it's fun, though I have no idea when I started doing it. I do remember that I didn't do it when I was a kid though.