What's taking so much space on my HD?

My machine came with a 120GB drive. I"m upgrading HD to 320Gb, I plan on using super duper to clone the drive etc., etc. Now, I previously had problems creating partitions larger than 3Gb with my current drive although the machine has almost 70Gb of free space. I understand that once i clone the drive all the free space on my new drive should be contigous thus eliminating the partition problem. My question is that how can I see what's taking up the 50Gb on my drive in order to do some "cleaning" before i clone the drive? My machine has barely been used and I dont see why I have used 50GB of space, My Itunes comprises of 4Gb and the rest is native apps with the exception of maybe 4 small 3rd party apps. Can u please recommend any software that I can use to see what is taking up space on my drive before I clone?