Nagios is probably the most famous and used monitoring program on the market. It's free, GPL and has nice features such as object representation of data, inheritance, plugin systems, passive testing, built-in Perl interpreter, result caching, pipe interface, alert delegations and so on and so on.
The web interface of Nagios is, however, incredible ugly. It's written in CGI the way the early CGI scripts were written. When you make a change to a server via the web interface, you get a few screens (avoiding Javascript is a benefit for some cell phones, I guess) and the old and quickly-annoying "You have done the action you wanted, please click this back link we created to go backwards" screen. You won't simply get automatically directed back to where you were before with a new message at the top in bright green saying "Action X done" or something like that. That would be too easy and Web 2.0. It uses frames (yuck!) to show the sidebar, you don't see the content of comments in hover, only when you click on the comment to get to the comment screen to view the comment. It's literally a pitfall and at least one company where I worked at rewrote the entire interface in ASP and .NET (I know, I know...) by parsing the Nagios log.
For that reason, it has always been a bit difficult (though possible) selling Nagios to the enterprise when your boss isn't tech savvy, and other programs, not much better or worse (OpenNMS for instance) find their way since they have a much better user interface.
A new fork of Nagios has begun with a PHP interface, calling Icinga. The point is to accept a lot of patches that were difficult to get into Nagios (that has only one actual developer - Ethan Galstad), and provide a beautiful web interface with Javascript. At least one Nagios community members sees the entire fork's point is the web interface, and assumes there's a good chance it will be merged back into Nagios, keeping the core as it is.
Apparently, there is a Perl Catalyst-based project to revamp the Nagios web interface, called Thunk. It is available on Github and also has a demo page. It's incredibly fast and seems promising. However, there is no website for it. Currently the only face it has is the Github page which seems generic and perhaps non-welcoming for some people who consider using it. You can also view the demo, but it still has the basic dull look of Nagios' oldschool interface.
Another new project relating to Nagios is Shinken, which is a Python rewrite of Nagios. Apparently someone thought that Nagios is great, except it's written in C, which makes it a barrier to include other peoples' work. I personally disagree for a variety of reasons which I won't go into. What does seem interesting is that Shinken is very welcoming, even though (at least to me) it seems like an exercise in futility. I'm assuming it will rapidly develop a stable core userbase, and the website is to thank, IMHO.
One more issue to note: Ethan Galstad is now developing Nagios XI, an enterprise solution. The "solution" boasts a new web interface, which I suspect will be the leading selling point of it.
So:
Now, of course there's a difference in Python, Perl, C and all that. However, in marketing, the selling value goes to the more presentable. In a System Monitoring conference I would have a pretty difficult time selling anything with just a Github page. It all boils down (in marketing terms) to the interface.
