web host monitoring

Hello Everyone,

    I have a question about monitoring networked hosts.  I am looking for something 
like Big Brother that monitors services on remote hosts.  I may end up using BB 
anyway, but I thought I'd ask for suggestions.
    I won't be able to run a client on some of the machines I want to monitor so 
the monitoring solution should not _require_ a client on the monitored host, but 
there may be a point in time where I do want to monitor some things that require 
a client on some of the hosts.
    Also, I am intersted in one that is really simple, just looking at WWW for a 
few hosts.  Something lightweight.

Thanks
Austin