Nagios works ok for me (& lots of others), but you'll get many other opinions/options ... Its like like asking for the 'best' Linux or editor...
With Nagios, the sysadmin sets it up, then the guys monitoring just look at a web page.
It'll do notifications via email or other methods if you have the infrastructure.
It shouldn't require much maintenance.
If you want external support, there are the Nagios Forums and loads of Docs/howtos
http://support.nagios.com/.
iirc, the 'Customer Support' link is for paying customers.
There are variations on the base install that come with (optional) paid support eg Groundwork, Centreon.
When I set it up, I looked at those but went with free Nagios Core install and used the free forums/google.
There's a lot of nagios users; its been around for a while and there are plenty of extra modules at
http://exchange.nagios.org/
HTH