Javascript-OmniCompletion-with-YUI-and-j is a project mainly written in Vim Script, it's free.
jQuery and YUI Omni Completion
This is a mirror of http://www.vim.org/scripts/script.php?script_id=3565
Dav Glass (https://github.com/davglass) started the modification. He added the YUI support, so I've decided to build on top of that foundation.
It far from perfect but it is usable.
Enjoy