I have been using Observium CE to monitor some devices and hosts on my network and have found Observium to be very easy to to install and use, and very valuable for keeping an eye on things.
The Community Edition ('CE') is free and open source. I started off using Observium to monitor some network devices only, but have recently added servers including my new Mango server. I run Observium itself in a small OpenVZ container on a separate physical server but you could probably run it on your Mango server if you had to. I thought I would share my experience for the interest of other Mango users.
The overview/dashboard page for my Mango server.
When you hover over anything, a detailed view will pop up:
The 'system health' page.
These pages are missing the menus and nav as I have had to delete some hovering menus and such from these pages when taking the screenshots.
Device storage detail:
Disk I/O detail:
Interface traffic page:
And, all these graphs will enlarge to a more detailed version when you hover the cursor:
There is a lot more that Observium can offer too. As you can tell, I'm very impressed.
To set all this up was as simple as installing and configuring snmpd on my server, and then adding the SNMP host details to Observium.
I am also monitoring a Windows server and this was also trivial to set up.