ext3cow is a time-shifting filesystem implemented with copy-on-write
(COW) and based on the very popular ext2/ext3 filesystem. It's a
functionally-enhanced version of ext3 that allows you to take a snapshot
of your filesystem with one second granularity, freezing the way the
filesystem looked at given point-in-time. You can then travel back
through time and be presented with a read-only image of the filesystem.
It needs no special mounting or "." directories; just type a name or
directory followed by the special "@" character and a time.