Almost every plugin and gem developed for Rails is available via GIT. Will_paginate is also there.

When we would like to install something under Windows OS, it can be slightly difficult.

When we execute something like this:

ruby script/plugin install git://nazwapluginu.git

It will fail. However adding slash after ".git" will fix this issue.

ruby script/plugin install git://nazwapluginu.git/

Edit:
Under Windows 7 slash is not needed any more :)