Bnf.vim is a project mainly written in Vim Script, it's free.
Highlight BNFs
This is a mirror of http://www.vim.org/scripts/script.php?script_id=250
Pretty straight forward. It highlights a BNF grammar definition. Not too fancy, but it makes writing a BNF more pleasant. You can see a screenshot of bnf.vim at http://students.cs.byu.edu/~mtb6/bnf_vim.png . For more complex BNF grammar definitions, you may want to check out http://vim.sourceforge.net/script.php?script_id=545 which is an EBNF syntax file by Hans Fugal.