This plugin causes trailing whitespace to be highlighted in red.

To fix the whitespace errors, call :FixWhitespace.  By default it
operates on the entire file.  Pass a range (or use V to select some lines)
to restrict the portion of the file that gets fixed.

The repo is at http://github.com/bronson/vim-trailing-whitespace

Originally based on http://vim.wikia.com/wiki/Highlight_unwanted_spaces

License: cc-by-sa.  (This may not be the best license for software but it
covered the instructions this plugin is based on. Until a reason to force
a different license appears, it's easiest to continue to use it.)

A more complete alternative is at https://github.com/ntpeters/vim-better-whitespace