Nagios is a full monitoring package you can run from a central server and see status via a browser.
NRPE is a plugin you can install on individual UNIX/Linux servers to monitor them. One of the binaries that comes with this plugin is "check_disk" which allows you to specificy warning and critical levels. In your nrpe.conf you would specify each partition you wish to monitor giving each the respective warning/critical levels you are interested in.
The main Nagios web site is:
http://www.nagios.org/
There is also even a plugin for Windows called "nsclient" that can be used for monitoring those systems.
We use the Nagios for monitoring CPU, Memory and disks, Oracle status, other processes. The beauty to Nagios is you can even write your own scripts to use within NRPE/Nagios to check various things.
For example I've written scripts to determine which node of a two node cluster is currently answering as the home of the "app". It shows a warning if failed over to the failover node and critical if down completely. We've also written monitors to use lynx to check status of web sites at a rudimentary level.
Even if you don't want to do the full Nagios setup you can get benefit by downloading the NRPE plugin and just running its command from the command line.
For example running:
/usr/local/nagios/libexec/check_disk -w 15% -c 5% -p /dev/sda7
Produced:
DISK OK - free space: / 5005 MB (83%);| /=1040MB;5138;5742;0;6045
In the first line we tell it to warn at 85% utilization (15% free) and go critical at 95% utilization (5% free). The partition is /dev/sda7.
The output shows "OK" (we haven't hit either warning or critical) along with the fact that it is using 5005 MB which is 83% utilization. It also figured out on its own that this is the root filesystem (/).
