Like "damn that's crazy", but when you want to be a little bit more family friendly.
Otherwise, when you care so little about what somebody is saying you have nothing else to say.
Jenny: "Oh my god, my day was so great! Me and becky we went out to the mall and blah, blah, blah..."
Tom: "Dang, that's crazy." (In other words, will this broad shut the fuck up?)