There aren't many cooler, yet legitimately-useful-to-science substances than liquid nitrogen. It's an instant ice cream maker's friend too, but did you know it makes for a really awesome bomb? Who said scientists don't know how to have fun?
Don't try this at home folks, that wicked liquid will freeze parts of you right off in an instant; you're rather attached to your digits and toes I'm assuming? Still, you could try* it with dry ice, but it's just not quite as spectacular. [Plymouth University via It's Okay to be Smart via io9]
*Don't try it at home folks, unless you want pieces of plastic shrapnel imbedded in parts of you.