latexdiff

Here's a great little tool: latexdiff. It takes two LaTeX files and produces a third LaTeX file showing deletions in struck-through red and additions in underlined blue:


Usage couldn't be easier:

latexdiff oldversion.tex newversion.tex > diff.tex
pdflatex diff.tex

If you run Ubuntu, you can get it with a simple "sudo apt-get install latexdiff".