As part of the Chaosdorf Admin Team, I recently wrote (and today overhauled) check_websites. You give it a URL and it checks if it is reachable under certain constraints.

All of this can also be achieved with check_http. However, since the Icinga configuration format is extremely verbose, I prefere to configure just one service which then uses check_websites to do all of this in one run.

Right now, it does actually not support that much. To quote its help:

-a, --auth=STRING
  Verify that URL requires auth (HTTP 401)
-o, --ok=STRING
  Verify that URL exists (HTTP 200)
-r, --redirect="FROM TO"
  Verify that FROM is a redirect (HTTP 301) to TO

In our case, we're calling it the following way:
/usr/local/lib/nagios/plugins/check_websites --extra-opts=@/etc/nagios/chaosdorf_websites.ini

On a side note: Nagios::Plugin is really, really awesome.