My hard drive is nearly full but I cant find whats taking up all my hard drive space

I have an intel macbook pro with 500 internal HD.

I have 160gb of music in my itues folder with another 7gb on a previous library, I am unable to find the size info of my applications folder but I susspect it to be around 50gb, however I only have 59 gb of free HD space remaining. How to I find out what is using the remainder od my hard drive space? I have not yet checked my iphoto. where would this be? Is there someway of checking to see what folders are the bigest on my hard drive?

I can's work out why my disk is so full.

Any help greatly appreciated. :-)