CSF do such a good work for monitoring users, running processes and etc... And i use Munin for monitor cpu usage and RAM usage. But it has more features than that Both of them are provided free!
If you want to monitor your server ports and all that from outside, then there are many of providers letting you so. I signed up for a one provider today. They offer a free account for monitor 1 server. But it's sufficient for me since i use 1 server
You can find them googling. I can't mention the name of the website or company. I'll dig into my memory and let you know if i can find that.