I had this problem the other day: my macbook pro was not sleeping when closing the lid.
I was 100% sure I have not modified any power settings nor installed new applications so I had to dig up a bit more.
Reading few websites about similar problems got me to a tool which helped me find the source of this problem.

The OS X releases come only via App Store as an app package but sometimes there is need for a way to transform this into an iso image.
So without further writing, I present you two ways to do so.

For several month I had this problem on one of my osx servers: the computer name was keep changing by adding a number at the end. Today was mmd (1), tomorrow mmd (2) and so on.
I have tried several things but nothing was successful until I finally gave up and looked up over the internet.
The solution was found here (link at the end of this article), among other methods. The one below was the most useful to me.

Php is able to read files and pass that information into a website. There are, of course, many ways to do this and below is just a tiny example.

Let's say you have in a text file, several lines in this format:

user@gzlinux $ cat files/servers-status.txt
server1#apache#10#up
server2#apache#20#down
server10#nfs#67#up
gzaix2#varnish#4#down
etc69#nginx#2#up

And you want to present this information in a php page with these tables: Hostname, Application, Number of Instances, Status.

No matter the monitorization solution, CPU utilization is important but if you really want to know what is happening with your system, you should look at load average. I will explain below why.

The current CPU utilization does not reflect the actual load of the system because when a host is heavily loaded, its CPU usage doesn't have to be necessary close to or at 100%.
Furthermore, the CPU utilization tends to generate (in monitoring tools) a significant number of false alerts, even if the percentage alert is close to 100%. Explanation follows.

There are many tutorials over the web regarding this problem but unfortunately it is not always the same for everybody.

In my case I have tried all the links below unsuccessfully (for you it might work):

How to get the Citrix XenServer version:

[root@XenGz ~]# cat /etc/redhat-release
XenServer release 6.5.0-90233c (xenenterprise)

[root@XenGz ~]# xe-linux-distribution 
os_distro="xenserver"
os_majorver="6"
os_minorver="5"
os_uname="3.10.0+2"
os_name="XenServer release 6.5.0-90233c (xenenterprise)"

Identify the uuid of the disk you want to attach but also the uuid of the target machine.

Citrix Xen has a way to migrate the machines from one host to another via xe vm-export and xe vm-import commands but there are situations when this won't work.

In this example, the VM name is "debian".

Get the DOM ID of your VM:

[root@xengz ~]# xe vm-list params=dom-id,resident-on name-label=debian
resident-on ( RO)    : 3664c511-cd7b-4705-8fcb-31f8b82d46d5
         dom-id ( RO): 3