If you want to go hardcore and build your own monitoring server, there is quite a bunch of free options. I was considering system monitoring solutions accompanied by Android Apps and Android home screen widgets:
Zabbix was my find #1, beyond Zenoss (which I am already familiar with). Zabbr (Zabbix Widget) is available at Google Play. If you want widget and a full scale app, then ZAX Zabbix Systems Monitoring can probably helpful. Overall there is quite a bunch of Zabbix apps, just search the Play Store for Zabbix.
Zenoss. If you want it free, look for Community edition. As for the app: Rhybudd - Zenoss for Android does have a neat widget putting Current Event Counts to your Android home screen. Requires special ZenPack. Zen Admin Core is another decent app for Zenoss management from Android is Zen Admin Core, it does not provide a widget.
Nagios - even though "nobody likes it" it is quite wide spread and has a great deal of Nagios Android apps on Play Store.
Last, but not least, I came across Icinga2. I did not find any dedicated Android apps for Icinga, but overall this monitoring system looks very interesting.