It's a cool and useful system monitor... and more.

Press ALT + F2 to see the command prompt, and then type "gnome-terminal" and hit ENTER.
Installation:
$ sudo apt-get update
$ sudo apt-get install conky conky-all
Aditional programs to check the processor temperature, etc:
$ sudo apt-get install curl lm-sensors hddtemp

curl: Improves the file transfers through the protocols HTTP,HTTPS and FTP.
lm-sensors: Temperature and state of the CPU, fans and the voltage.
hddtemp: HDD temperature.

Run the application as program:
$ conky

Stop the application:
$ pkill conky

Run the application as a daemon:
$ conky -d

$ ps -aux | grep 'conky'
Output:
esteban   4640  0.4  0.1 516912  1456 ?        Sl   14:42   1:54 conky -d
esteban   7862  0.0  0.2  12724  1952 pts/3    S+   21:40   0:00 grep conky

Kill the daemon:
$ kill 4640

Create the file /home/esteban/.conkyrc and add the custom configuration for your user session:
$ touch /home/esteban/.conkyrc
$ gedit /home/esteban/.conkyrc

File .conkyrc:

# Data position, size, etc.
background no
font Snap.se:size=8
xftfont Snap.se:size=8
use_xft yes
xftalpha 0.1
update interval 3.0
total_run_times 0
own_window yes
own_window_type normal
own_window_transparent yes
own_window_hints undecorated,below,sticky,skip_taskbar,skip_pager
double_buffer yes
draw_shades no
draw_outline no
draw_borders no
draw_graph_borders no
minimum_size 200 6
maximum_width 300
default_color ffffff
default_shade_color 000000
default_outline_color 000000
alignment top_right
gap_x 10
gax_y 10
no_buffers yes
cpu_avg_samples 2
override_utf8_locale no
uppercase no
use_spacer no
own_window_argb_visual yes

TEXT
# Configuration of the data to show
# System name and kernel version
${font Debian:style=bold:size:12}$sysname $alignr $kernel

# Information about the processor and usage status bar
${font Debian:style=bold:size=12}Processors $hr
${font Debian:style=bold:size=8}CPU1: ${cpu cpu1}% ${cpubar cpu1}
CPU2: ${cpu cpu2}% ${cpubar cpu2}

# Processor temperature
Temperature: $alignr ${acpitemp} C

# Status of the main partitions, RAM and swap
${font Debian:style=bold:size=12}HDs, RAM and swap  $hr
${font Debian:style=bold:size=8}HOME $alignr ${fs_used /} / ${fs_size /} 
${fs_bar /}
${font Debian:style=bold:size=8}RAM $alignr $mem / $memmax
${membar}
${font Debian:style=bold:size=8}SWAP $alignr $swap / /$swapmax
$swapbar

# Status of the network interfaces
${font Debian:style=bold:size=12}Network interface $hr
${font Debian:style=bold:size=8}Down $alignr ${downspeed eth0}/s
${downspeedgraph eth0 30,210 01df01 10fd10}
${font Debian:style=bold:size=8}Up $alignr ${upspeed eth0}/s
${upspeedgraph eth0 30,210 0000ff ff0000}

# Usage of CPU
${font Debian:style=bold:size=12}CPU usage $hr
${font Debian:style=bold:size=8}${top name 1}$alignr${top cpu 1}%
${top name 2}$alignr${top cpu 2}%
${top name 3}$alignr${top cpu 3}%

# Usage of RAM
${font Debian:style=bold:size=12}RAM usage $hr
${font Debian:style=bold:size=8}${top_mem name 1}$alignr${top_mem mem 1}%
${top_mem name 2}$alignr${top_mem mem 2}%
${top_mem name 3}$alignr${top_mem mem 3}

End of the file.


Start Conky again to see the new changes:
$ conky -d

Add to the main menu:
Gnome > Applications > Main Menu
Command: conky -d
Icon: /Usr/share/icons/gnome/48x448/mimetyes/appication-x-addon.png

If you want to add Conky as a startup App, you can choose between:
Add the program as a service to systemd or sysvinit
Add the program as a startup app in Gnome or Fluxbox
See my Screensaver guide here for details:
References:
Guide: Xscreensaver

Skins and themes:
Reference:
Web: http://www.noobslab.com/2012/07/conky-collection-for-ubuntulinux.html

For example, I am gong to install "Conky Lua":
Web: http://www.noobslab.com/2012/06/install-conky-lua-in-ubuntulinux.html
Steps:
$ cd && wget -O lua-noobslab-gray.zip http://drive.noobslab.com/data/conky/Lua/conky-lua-gray.zip
Unzip the files into /home/esteban, where the old .conkyrc resides:
$ unzip conky-lua-gray.zip
drwxr-xr-x  2 esteban esteban 4.0K Jul 12  2012 .conky
-rw-r--r--  1 esteban esteban 2.5K Mar 18  2013 .conkyrc
If the program was turned off, just type ALT + F2 and start the program.

Conky Manager:
You can try to donwload the program from the PPAs of Ubuntu and build the program from source. You can learn to build packages from source with this guide:
References:
Guide:
Build packages from source
SimpleScreenRecorder

I've found this web site and decided to download a version of the package from here: http://www.teejeetech.in/2014/06/conky-manager-v2-released.html
Anyway, I don't recommend to donwload packages this way, because we can't certificate the providers, who can be you and me.
Run the program:
$ conky-manager
The program is going to show up in the Gnome apps menu.