MarkLines is a project mainly written in Vim Script, it's free.
Toggle highlighting on important lines
This is a mirror of http://www.vim.org/scripts/script.php?script_id=2028
The MarkLines plugin allows one to visually mark any number of lines, in any number of buffers, with any chosen highlight. In a sufficiently new version of VIM, 7.1.040 or greater, you can use more than one highlight color per window simultaneously.
THIS PLUGIN REQUIRES VIM 7.0 OR GREATER.
Useful for things like 'Mark every line that contains the word...' or 'Brighten every line containing a function declaration' without having it cleared every time you search for something else. :)
(By the way, you could do the first example with :%g/foo/MarkLinesOn)
:MarkLinesOn turns on highlighting for the current line or range :MarkLinesOff turns off highlighting for the current line or range :MarkLinesToggle toggles highlighting for the current line or range
Each can be followed by an optional highlight group name as an argument; if so they turn on highlighting to that color (On, Toggle) or only clear lines of that color (Off).
A few maps are provided as well.