How to make PyCharm always show line numbers

Version 2.6 and above:
PyCharm (far left menu) -> Preferences... -> Editor (bottom left section) -> General -> Appearance -> Show line numbers checkbox

Version 2.5 and below:
Settings -> Editor -> General -> Appearance -> Show line numbers checkbox

For version 4.0, 4.5 on Windows

File -> Settings

Then,

Editor -> General -> Appearance -> Show line numbers

For version 4.0 on Mac OSX

PyCharm-->Preferences

Then,

Editor-->General-->Appearance-->checkbox: "Show line numbers"

For version 3.0 (Community Edition):

File -> Settings -> Editor (under IDE Settings) -> Appearance -> check 'Show line numbers'