This project appears to be no longer maintained.

For natty there is a touchpad indicator app. it can be set to disable/enable the touchpad when a mouse is plugged in or out. I have no idea if it might also work for older Ubuntu versions.

https://launchpad.net/touchpad-indicator

sudo add-apt-repository ppa:atareao/atareao
sudo apt-get update
sudo apt-get install touchpad-indicator

You can set up udev rules to disable/enable touchpad. ArchLinux wiki has these rules for toggling a Synaptic touchpad from udev:

ACTION=="add", SUBSYSTEM=="input", KERNEL=="mouse[1-9]", ENV{DISPLAY}=":0.0", 
ENV{XAUTHORITY}="/home/USERNAME/.Xauthority", ENV{ID_CLASS}="mouse", RUN+="/usr/bin/synclient TouchpadOff=1"

ACTION=="remove", SUBSYSTEM=="input", KERNEL=="mouse[1-9]", ENV{DISPLAY}=":0.0", 
ENV{XAUTHORITY}="/home/USERNAME/.Xauthority", ENV{ID_CLASS}="mouse", RUN+="/usr/bin/synclient TouchpadOff=0"

I'm not sure if all the laptops use Synaptiks, but you could try to use it. In case you haven't installed, write in a terminal: $sudo apt-get install kde-config-touchpad, then $synaptiksand finally in the menu «Touchpad management» check:

• Automatically start at logon
• Automatically switch off touchpad, if a mouse is plugged

I hope this information has been valuable for you!

You can create a script, and execute it when you want to disable/enable touchpad.

/usr/local/bin/touchpad_toggle.sh

#!/bin/bash

declare -i ID
ID=`xinput list | grep -Eio 'touchpad\s*id\=[0-9]{1,2}' | grep -Eo '[0-9]{1,2}'`
declare -i STATE
STATE=`xinput list-props $ID|grep 'Device Enabled'|awk '{print $4}'`

if [ $STATE -eq 1 ]
then
    xinput disable $ID
    # echo "Touchpad disabled."
    # notify-send 'Touchpad' 'Disabled' -i /usr/share/icons/Adwaita/48x48/devices/input-touchpad.png
else
    xinput enable $ID
    # echo "Touchpad enabled."
    # notify-send 'Touchpad' 'Enabled' -i /usr/share/icons/Adwaita/48x48/devices/input-touchpad.png
fi

This is a more lengthy answer of similar one by realhu.

Script to toggle Touchpad on/off with screen notification

Partial credit to this post (Enable/disable touchpad)

Create toggle-touchpad script

Create a new directory /home/USER/bin and then use gedit /home/USER/bin/toggle-touchpad. NOTE: Replace USER with your user ID. Copy and paste these lines into your editor:

#!/bin/bash

# NAME: toggle-touchpad
# PATH: /home/$USER/bin
# DESC: Update pulseaudio output device when HDMI TV plugged / unplugged
# CALL: called from Keyboard Shortcut `Super`+`T`
# DATE: Created Dec 23, 2016.
# NOTE: Written for AU question: http://askubuntu.com/questions/863746/keyboard-shortcut-to-disable-the-laptop-touchpad/863750?noredirect=1#comment1333958_863750


# Use device number matching touchpad, in this case 14
if [[ $(xinput list 14 | grep -Ec "disabled") -eq 1 ]]; then
    xinput enable 14
    DISPLAY=:0 notify-send --urgency=critical --icon=/usr/share/icons/gnome/256x256/status/user-available.png "Touchpad enabled"
else
    xinput disable 14
    DISPLAY=:0 notify-send --urgency=critical --icon=/usr/share/icons/gnome/256x256/status/user-busy.png "Touchpad disabled"
fi

exit 0

Mark toggle-touchpad script as executable

Save the file and exit the editor. Now flag the file as executable using

chmod +x /home/USER/bin/toggle-touchpad

Assign toggle-touchpad script to keyboard shortcut

Open up System Settings ⟶ Keyboard ⟶ Shortcuts ⟶ Custom Shortcuts ⟶ +

This screen appears:

Fill in the Custom Shortcut fields like this:

• Name = Toggle Touchpad
• Command = /home/USER/bin/toggle-touchpad

Click Apply button to save.

The new entry appears with status Disabled. Right click on Disabled and use Super+Z (or any other unused shortcut combination). I wanted to use Super+T but that is already assigned to Nautilus Trashcan.

Modify toggle-touchpad script to different device number

The default device number is set at 14. To find out what your device number is use the following:

───────────────────────────────────────────────────────────────────────────────
USER@host:~/bin$ xinput
⎡ Virtual core pointer                      id=2    [master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer                id=4    [slave  pointer  (2)]
⎜   ↳ Logitech Performance MX                   id=10   [slave  pointer  (2)]
⎜   ↳ Logitech K800                             id=11   [slave  pointer  (2)]
⎜   ↳ AlpsPS/2 ALPS GlidePoint                  id=14   [slave  pointer  (2)]
⎣ Virtual core keyboard                     id=3    [master keyboard (2)]
    ↳ Virtual core XTEST keyboard               id=5    [slave  keyboard (3)]
    ↳ Power Button                              id=6    [slave  keyboard (3)]
    ↳ Video Bus                                 id=7    [slave  keyboard (3)]
    ↳ Power Button                              id=8    [slave  keyboard (3)]
    ↳ Sleep Button                              id=9    [slave  keyboard (3)]
    ↳ Laptop_Integrated_Webcam_HD               id=12   [slave  keyboard (3)]
    ↳ Dell WMI hotkeys                          id=15   [slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard              id=13   [slave  keyboard (3)]
───────────────────────────────────────────────────────────────────────────────
USER@host:~/bin$ 

You can pick any device you like, ie Touchpad = 14, Webcam = 12, etc.

Which ever device number you use, simply open your /home/USER/bin/toggle-touchpad script and replace 14 with that device number.

Modify toggle-touchpad script to use different icons

When the "Touchpad enabled" / "Touchpad disabled" notification bubble is displayed, an icon is displayed left of the text. Stock icons are used from /usr/share/icons/gnome/256x256/status/ but you can change them.

For enabling touchpad this is displayed:

For disabling touchpad this is displayed: