Simple monitoring solution

I've seen quite a few people use NewRelic

If you need something more minimalistic, something to check your resource usage during certain times of the day, Linode Longview accomplishes that.

You can also set your Linode Dashboard to notify you if you're using passed a certain threshold:

Hope this helps!

It depends what you mean by "monitoring." I like monit for process supervision - that is, performing periodic checks to make sure a service is working correctly and taking action if it isn't. Among the things it can check are CPU usage, memory usage, and responsiveness over the network. The actions taken can be an alert (e-mail) or any arbitrary shell command (such as restarting a service). It is very lightweight and quite flexible, and many distributions include it in their repositories. However, monit does not collect statistics, so if you're looking for something to generate graphs you should look elsewhere.

It should be noted that systemd seems to be targeted at doing a lot of the process supervision tasks that monit performs. I have never used monit in conjunction with systemd, and suspect that it might be tricky to use them together.