Mac OSX TimeMachine's "local snapshots" are basically backup files stored on your local hard-disk (as opposed to regular ones stored on the external TimeMachine server). This is a useful feature for people who travel a lot, so they can revert files to recent versions even when they are not connected to their home TimeMachine.
Unfortunately, this feature tends to take a lot of disk space whenever you deal with big files (like movies etc). In some cases, you might want to temporarily disable it in order to claim back a few GigaBytes. Note that your regular TimeMachine backups will NOT be affected; you'll just lose the ability to revert to recent (i.e. from last week) versions without being connected to the external TimeMachine.
TimeMachine is managed from the command line with the tmutil command. You can type man tmutil to see all options. To disable the snapshots and delete those big files (which are found under /.MobileBackups, by the way), use "sudo tmutil disablelocal". Once snapshots are purged, you can restart the feature (if you need it) with "sudo tmutil enablelocal" (yeah, not exactly rocket science).
In my case, this procedure freed some 139 GB on a 512 GB disk. Not bad!
