Depends what you're studying probably. For reading notes and such music can be pretty distracting unless you go for something ambient/classical. When it comes to solving problems/answering questions (the best form of studying for anything math) then pretty much any music will do.
How are people even able to study with music °°___
http://www.youtube.com/watch?v=cUvEZgQC-tY
If that's not chilled I don't know what is.