This plugin causes all trailing whitespace to be highlighted in red.

To fix the whitespace errors, just 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

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