Conky is a light-weight system monitor.

Conky’s Homepage: http://conky.sourceforge.net/index.html

It can be easily customized by editing ~/.conkyrc, and the following resources may be quite useful:

http://conky.sourceforge.net/config_settings.html

http://conky.sourceforge.net/variables.html

Here is a sample .conkyrc file:

# Conky, a system monitor, based on torsmo
#
# Any original torsmo code is licensed under the BSD license
#
# All code written since the fork of torsmo is licensed under the GPL
#
# Please see COPYING for details
#
# Copyright (c) 2004, Hannu Saransaari and Lauri Hakkarainen
# Copyright (c) 2005-2010 Brenden Matthews, Philip Kovacs, et. al. (see AUTHORS)
# All rights reserved.
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.
#

alignment top_right
background no
border_width 0
cpu_avg_samples 2

default_color white
default_outline_color white
default_shade_color white
draw_borders no
draw_graph_borders yes
draw_outline no
draw_shades no

use_xft yes
xftfont WenQuanYi Micro Hei Mono:bold:antialias=True:pixelsize=13

gap_x 10
gap_y 10
minimum_size 5 5
net_avg_samples 2
no_buffers yes

out_to_console no
out_to_stderr no
extra_newline no

own_window yes
own_window_class Conky
own_window_colour 333333
own_window_type desktop
own_window_transparent no
own_window_argb_visual yes
own_window_argb_value 127

stippled_borders 0
update_interval 1.0
uppercase no
use_spacer none
show_graph_scale no
show_graph_range no
double_buffer yes

TEXT
${color green}${alignc}$nodename - $sysname $kernel on $machine
${color #6699cc}$hr
${color #6699cc}${alignc}${scroll 40 $mpd_artist - $mpd_title}
${color #6699cc}$mpd_bar
${color #6699cc}${alignc}$mpd_status
${color #6699cc}$hr
${color green}Uptime:${color yellow} $uptime
${color green}Frequency:${color yellow} $freq_g GHz
${color green}CPU temperature: ${color yellow}${acpitemp}℃
${color green}Battery: ${color yellow}${alignr}$battery_percent%
${battery_bar}
${color green}RAM:${color yellow}  $mem/$memmax ${alignr}$memperc%
${membar 4}
${color green}Swap:${color yellow} $swap/$swapmax ${alignr}$swapperc%
${swapbar 4}
${color green}CPU1 ${alignr}CPU2
${color yellow}${cpugraph cpu1 40,180 blue red} ${alignr}${cpugraph cpu2 40,180 blue red}

${color green}/     ${color yellow}${fs_used /}/${fs_size /} ${fs_bar 6 /}
${color green}/home ${color yellow}${fs_used /home}/${fs_size /home} ${fs_bar 6 /home}

${color green}eth0 ${color yellow}Up:${upspeed eth0} Down:${downspeed eth0}
${color green}tun0 ${color yellow}Up:${upspeed tun0} Down:${downspeed tun0}

${color green}Processes:${color yellow} $processes  ${color green}Running:${color yellow} $running_processes
$color$hr
${color green} Name              PID    CPU%   MEM%
${color green} ${top name 1} ${top pid 1} ${top cpu 1} ${top mem 1}
${color green} ${top name 2} ${top pid 2} ${top cpu 2} ${top mem 2}
${color green} ${top name 3} ${top pid 3} ${top cpu 3} ${top mem 3}
${color green} ${top name 4} ${top pid 4} ${top cpu 4} ${top mem 4}

Note. To add real transparency, first you need a compositing manager such as xcompmgr. Then you need to add the following options in ~/.conkyrc:

own_window_transparent no
own_window_argb_visual yes
own_window_argb_value 127

onw_window_transparent is set to no, otherwise the window will be fully transparent. own_window_argb_visual must be turned on. own_window_argb_value is a value between 0 and 255 to indicate how much transparency you want.