RltvNmbr.vim is a project mainly written in Vim Script, it's free.
Display relative line numbers
This is a mirror of http://www.vim.org/scripts/script.php?script_id=2351
The RltvNmbr plugin uses the signs feature to display the line numbering relative to the current line. A number of commands take [count] arguments for which this is helpful.
:RltvNmbr This command turns relative line numbering on.
:RltvNmbr! This command turns relative line numbering off.
CAVEAT: Your vim must have the signs feature compiled in! This typically means that your vim has been configured and compiled to be "huge": configure --with-features=huge compile install
To check this:
:echo has("signs")
should show a "1". Alternatively, look at the output of :version and see if you have +signs listed.
(alpha/beta version available at http://mysite.verizon.net/astronaut/vim/index.html#RLTVNMBR)