Yesterday I was cleaning off my HD getting ready to partition it so that I could install the Lion beta. At the time I had 70 GB free and I was a little bummed that my Lion partition was going to need to be small. I started using the excellent DaisyDisk tool to analyze what was eating all of my free space and found a culprit buried deep inside (not really) the Application Support folder.
This folder was taking an amazing 50GB of space.
The folder is in your user > Library > Application Support > MobileSync folder and is called ‘Backup’. I am wary of deleting any folder with the title ‘Backup’ and some research on DuckDuckGo and Twitter turned up that this is the folder iTunes uses to store iPhone backups. Mine is so large because of all the iOS beta builds that I use. I ended up deleting a bunch of the old backups (you can read the Info.plist file to see what device and sync date the file is for), but I still had over 8 GB of data in the folder.
I don’t keep my iTunes library on my Mac, I keep it on an external drive, which means that when I sync I need to be plugged into that drive to begin with. So I wanted to move the Backup folder off of my Mac and onto that drive — luckily you can do this with a simple symbolic link and the entire operation is detailed here.
If you are low on space I recommend taking a look at this — might save you a few GBs.
[Updated: 2.25.11 at 10:03 AM]
You can also delete these backups in your iTunes prefs, but I recommend getting the folder off your Mac if you can.