wp-propagator is a WordPress plugin by aaires and nunomorgadinho will keep track of what plugins are installed and will copy and activate them to a number of other WordPress installs.
It is useful for when you have a set of separated WordPress installs which you want to keep synchronized in terms of plugins.
It does not require XML-RPC enabled or FTP support since the requests are done via HTTP POST requests.
Download and info: http://wordpress.org/extend/plugins/wp-propagator/