How do you monitor Running Services on Linux boxes?

Hi,

There seems to be a few different options for monitoring services on Linux machines.  We had been using one that uses SNMP, but it's been giving us trouble with some machines showing No Data every so often.  We've also had the check start ignoring services when they stop and then removing the instance.

We recently started trying an SSH based check which seems to work.  However, it's based on setting a property on every machine/group to tell it which services to monitor.

I'm just curious what module other people use to monitor services like this with the most reliable results.

Thanks.