What's taking up all my hard drive space?

My daughter has a MacBook with a 100GB hard drive. Since OS X takes 20%+ of that, effectively she has about 76GB to play with. She's only got 3GB free space left so the machine has slowed to a crawl. The thing is, I can't track down what's taking all the space. She has about 7GB of music, under 5GB of images, nothing much in the way of documents, no heavyweight apps, nothing in Trash. From a quick eyeball I'd reckon she should only have used about 60% of her hard drive space max, including OS X. I've run all the Onyx maintenance routines and got no additional space as a result.

Is there a utility or app that will tell me what's using what so I can try and sort this out for her?

From what you've described (at a glance), she should have at least 65+GB free!
An easy to use app called disk inventory x
Disk Inventory X

Other option is to go to HD, and Home folder (daughter's folder) in finder, right click on each item/get info and trace it that way. Doesn't take long to see by just right clicking on each enclosing folder and getting that info.

Also, you didn't mention movies in your post...... nothing there?