the changebar package allows marking changes by bars. Not automatically, it has to be done by commands:\cbstart and \cbend. Change may can be nested within each other and with other environments, they can be colored and can cross page boundaries.
Latexdiff is a Perl script for visual mark up and revision of significant differences between two latex files. Various options are available for visual markup using standard latex packages such as color . Changes not directly affecting visible text, for example in formatting commands, are still marked in the latex source. A rudimentary revision facilility is provided by another Perl script, latexrevise, which accepts or rejects all changes. Manual editing of the difference file can be used to override this default behaviour and accept or reject selected changes only.