i had the same problem with my 128GB hard disk MacBook Air too. I used Finder to identify that it's my Users folder that contained invisible folders or files excessive in sizes. They accounted for some 80GB of the hard disk while the sizes of files in viewable folders totaled less than 6GB (including music and photo files). The culprit folders and files must have been hidden within the Users folder, I concluded.
I downloaded TinkerTool successfully - be sure to download the corrrect version to match your Mac OS X version - and in thet General tab, I selected "Show hidden and system files", then I selected "Relaunch Finder" and using the relaunched Finder I was able to see a hugh hidden folder named "Library" within my Users folder. Probing further, I discovered hugh back up files in a "Back Up" folder within an "Application Support" folder inside the "Library" folder. The back up files numbered in the ten thousands, had cryptic file names, but were viewable by opening them with Textedit. I decided I don't need them and trashed the "Back Up" folder files to recover 70+GB.
I read from TinkerTool FAQ that the Mac OS X originated from NeXT so many of the advanced file management features have been "deactivated" for the general user migrating up to OS X from the previous Mac OS. The simplicity, cleaniness, and user friendliness have been retained. It thus takes the use of tools like TinkerTool to accomplish additional tasks at times.
Thanks all for the TinkerTool tips above! Just to share my own success story.