It could be very usefull to monitor an external (non-SCM'ed) file or http-url
for modification as build trigger. This will be used to monitor some kind of
external dependency, for instance from a maven repository. The feature should
check the timestamp of the file, not the contents. For http urls this is
normally performed using an http HEAD request.

Right now we have more projects and modules that can be built using a single
hudson instance. This feature would make it trivial to set up inter-hudson
module dependencies.