Two different definitions of cool!

First: "of or at a fairly low temperature". It snowed today... It will get warmer according to the forecast, but I'm having to wear several layers to keep warm today. Quite cool.

Second: "informal excellent". Michal and Martina, two of the OMers have just been to Egypt and gave me a stone from Mount Sinai. Very cool.

Definitions taken from: http://www.askoxford.com/