A light alternative to gnome-system-monitor?

htop

it is a terminal app:

sudo apt-get install htop
htop

_{htop with graph mode}

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ProcMeter3

Look for this system monitor (GUI works with GTK1,2,3):
ProcMeter3

An utility which works with LCD devices even.

_{ProcMeter3 GUI on GTK3}

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Psymon

Also once I tried a Psymon. This is not extremely lightweight software and written
on Qt (not GTK, and therefore should work better with KDE) though despite to this fact it works well not only with Unity and Gnome, but also in other OS, like FreeBSD, MacOS, Windows etc., because it use great python-psutil Debian package.

So look it closer:

Nice post about Psymon

Psymon project site

_Psymon

You can try

• xfce4-taskmanager from Xfce or
• lxtask from LXDE.

Neither of them pull any specific dependencies.

ndicator-SysMonitor Indicator-SysMonitor does a little, but does it well. Once installed and run, it displays CPU and RAM usage on your top panel. Simple.

Download from here

Conky

One of my personal favourites

Screenlet you’ll find a bunch of differently styled CPU and RAM monitors included in the screenlets-all package available in the Ubuntu Software Center.

Glances

To install:

sudo apt-get install python-pip build-essential python-dev
sudo pip install Glances
sudo pip install PySensors

VMSTAT

Displays information about CPU, memory, processes, etc.

IOSTAT

This command line tool will display statistics about your CPU, I/O information for your hard disk partitions, Network File System (NFS), etc. To install iostat, run this command:

sudo apt-get install sysstat

To start the report, run this command:

iostat

To check only CPU statistics, use this command:

iostat -c

For more parameters, use this command:

iostat --help

MPSTAT

The mpstat command line utility will display average CPU usage per processor. To run it, use simply this command:

mpstat

For CPU usage per processor, use this command:

mpstat -P ALL

Saidar

Saidar also allows to monitor system device activities via the command line.

You can install is with this command:

sudo apt-get install saidar

To start monitoring, run this command:

saidar -c -d 1

Stats will be refreshed every second.

GKrellM

GKrellM is a customizable widget with various themes that displays on your desktop system device information (CPU, temperature, memory, network, etc.).

To install GKrellM, run this command:

sudo apt-get install gkrellm

Monitorix

Monitorix is another application with a web-based user interface for monitoring system devices.

Install it with these commands:

sudo add-apt-repository ppa:upubuntu-com/ppa
sudo apt-get update
sudo apt-get install monitorix

Start Monitorix via this URL:

http://localhost/monitorix/