For a really simple solution at the Terminal:
% tail -f log_file.log
From the tail mainfile (i.e.
-f The -f option causes tail to not stop when end of file is reached,
but rather to wait for additional data to be appended to the input.
The -f option is ignored if the standard input is a pipe, but not if
it is a FIFO.
-F The -F option implies the -f option, but tail will also check to see
if the file being followed has been renamed or rotated. The file is
closed and reopened when tail detects that the filename being read
from has a new inode number. The -F option is ignored if reading
from standard input rather than a file.