Looks very promising - I can't help wonder how you can spend time building a quality product like this, then release it for free : is there a monetization strategy at some point (i.e : pro support) ?
There is a Pro version in the works.
The idea is to have a fully working open source variant with almost everything included. But if you want to monitor multiple servers, better support for multiple apps ( for example: color filter in the web interface ), email notifications , etc - then you can buy the Pro version.
The Pro version will cost something that everyone can afford and it will be - pay once, use forever :)
Don't hesitate to charge a reasonable monthly fee. There's a lot of room to undercut the existing players (newrelic et al) and still make it a sustainable business.
Maybe I will do that, because it will be more convenient for some people. But the Pro version will always be an option for people that don't want to pay monthly charges and are OK with managing their own instances :)
P.S And of course the Open Source version will do the job for most people