I am surprised no one has recommended the original Big Brother from 
http://www.bb4.org/download.html - it's a pretty easy to set up, so 
don't overlook it.
The install is simple, you compile a monitoring daemon and a reporting 
client, then the rest is just shell scripts that execute the client to 
report back to the daemon.
If you can write a shell script that checks what you want to check, you 
can monitor it with bb4.  It supports ping and http checking out of the box.
Unless I'm missing the boat here, If you're looking for a simple but 
expandable/customizable monitoring system, this is it.
I have found that at least for me, Nagios is quite complex in comparison 
as far as creating a configuration.  You can generally have a bb4 
monitoring system set up in just a few minutes after RTFM.
It can email alerts, has a pretty sophisticated alert 
scheduling/exclusion tool, has a summary/status website, and can run an 
external script to perform notification in other manner such as dialing 
a modem, etc.
The newer "professional edition" costs money but as far as I know the 
older version 1.9 is still free for "non-commercial" use. 

Disclaimer:  I don't work for BB4, but I have used it for many years.  
Ginsu knives sold separately, YMMV, etc, etc.

Best Regards,
Jesse

>>>>>>>  On 4/1/07, Dan Trevino <[log in to unmask] > wrote:
>>>>>>>               
>>>>>>>> Does anyone have experience with any of the many free website
>>>>>>>> monitoring services available?