How to open system monitor easily?

Run in terminal,

gnome-system-monitor

This would open system monitor.

GNOME System Monitor

Run the system monitor from Terminal by typing: gnome-system-monitor

Alternatively, run this command by pressing ALT+F2 (or Super, also known as "Windows key") and typing gnome-system-monitor

Keyboard shortcut

You can add a custom keyboard command using the following steps:

1. Go to System Settings
2. Click Keyboard
3. Open the Shortcuts tab
4. Click the "+" symbol near the bottom of the window
5. Give your command a title. E.g. System Monitor
6. In the Command field, enter gnome-system-monitor
7. Click Apply
8. Click Disabled on your newly created shortcut
9. Enter the key combination you want for your shortcut. E.g. Ctrl+Alt+M

Note: on Ubuntu 14.10, Ctrl+Alt+Del is already in use, but can be overridden.

Why open the terminal at all? You are opening an extra application, which you may not be able to close without closing the System Monitor, to open something that can be done with some keystrokes: Alt + F2 then type gnome-system-monitor. That's the most hassle free.