Yeah even if you've got GigaBytes of it you still seem to run out! Here's a few free tools that are good for visualising used space - using a nice visualisation technique known as treemaps:
Windows: WinDirStat, SequoiaView (a bit old)
Mac OSX: Disk Inventory X, GrandPerspective. (Baobab is available thru darwinpaorts)
Linux: Disk Usage Analyzer (aka Baobab)
There's also plenty of others out there but I've not tried 'em.