Hello,
I have an Ubuntu web server with Plesk control panel installed. It comes with a nice mobile app as part of a "Power Pack" or something, costs extra. I also have another Ubuntu machine I'd like to monitor with nothing special installed.

Is there something already out there that you would recommend that I could tap in and get a pulse of how things are running with an API? I'd love to feed it into a custom HTML5 app of some kind that I create, mixed in with stats from other APIs. JSON is a great data format I'm getting from all the other APIs I'm working with.

I'd really like to get memory usage, average CPU load, available storage space. That's about it (although I'd make use of anything extra).

Here's what I've found so far:
http://sebastien.godard.pagesperso-orange.fr (not sure how to interface with this yet, maybe I could create a Python web server running on some port that makes use of this? - the website is very nostalgic!)

http://sensuapp.org/ (Very cool looking site... it seems like I have to install and maintain a lot of stuff to make this work?)

I also thought about learning node.js and doing something with that (but on the server monitoring front, that's another rabbit hole...)

I'd enjoy hearing what your think. I appreciate your feedback in advance.

Install and run SNMPD on your server and use any standard SNMP client to interrogate it.