This Apache module is intended to reduce the load on your server & bandwidth
generated by popular virtual hosts, directories, locations, or users according
to supported polices that decide when to delay or refuse requests. Also
mod_throttle can track and throttle incoming connections by IP address or by
authenticated remote user.

Every request now passes through four levels of throttling, which are: by
client's IP address , by authenticated remote user name (ThrottleRemoteUser),
by local user ID (ThrottleUser), and by directory, location, virtual host, or
server (ThrottlePolicy).

WWW: http://www.snert.com/Software/mod_throttle/