Paraphrasing since(1):

  since is similar to tail(1) since it also displays the tail of a
  file.  However, since displays only the data which has been added
  since the last time since was run.  If since has not been run
  before, the entire file is displayed.  since is useful for watching
  over log files.

WWW:  http://welz.org.za/projects/since