If you use an SSD -- storage space is a precious asset, and you have to manage it wisely. Today I recovered 50GB of space on my SSD with the help of Tjark Derlien's excellent (and free) Disk Inventory X which shows you what's filling your SSD (or hard drive) in an organized, color-coded graphic.
If you're rocking a large hard drive (like the 750GB and 1TB mechanisms available for the MacBook) then you're relatively immune to the problem. SSD owners, on the other hand, need to be especially aware of space management issues and gigabyte-hogging applications. No one know this better than MacBook Air owners.
My problems began when I started getting "startup disk full" warnings and when my VMWare image would force suspend for lack of space. I immediately fired up Disk Inventory X to find the source of the bloat and here's what I saw:
As I was mousing around in the big blue area in the upper left I noticed that a full fifth of my SSD (50GB out of 250GB) was being clogged with a folder located here:
(That other massive chunk on the right in red, incidentally, is my VMWare Fusion image for Windows 7.)
After scrambling to delete files the last time I ran out of space -- only to save a couple gigabytes -- I discovered the mother load.
The MobileSync/Backup folder is used to store your iOS backups. Every time your iPhone, iPad or iPod makes a backup via iTunes, it's stored in the MobileSync/Backup folder. If you have a 32GB iPhone and a 32GB iPad, this can add up fast. If you frequently backup and restore your iOS devices (I'm talking to you, developers) the Backup folder can get unwieldy pretty fast.
The Apple prescribed way to move MobileSync Backups (as outlined in knowledge base article HT1766) is to select iTunes > Preferences > Devices and delete unneeded backups.
Boom, 50GB recovered.
I made sure to make fresh backups my iPhone and iPad after doing deleting the folder, but I managed to trim my Backup folder from 50 to 2GB simply by archiving the old backups.
Apparently, there's no way to make iTunes write the backups to another disk, but I'm looking into that...