I need to ensure that any number of websites and services haven't bitten the dust. At the moment I rely on a simple 5 minute cronjob which uses curl and a file of urls to check that all the responses are correct (200 or rarely 301).
It all works on a basic level, I'm interested in more sophisticated open-source tools (statistics, response times, pretty graphs) - what do you use?
In my experience nagios works pretty well but seems to be hard to take care of at times. If you want to have stats and everything else on top of what nagios gives you then add cacti. If you want it all in one package you should look into OpenNMS. To get the full benefit of all of these you should be using SNMP too so you can see the details of memory, cpu, disk use as well.