A plugin that does away with X-Plane's idiotic centered little box for mouse steering and replaces it with a more sane system for those who, for whatever reason, want to or have to use the mouse for flying.
The plugin removes the centered + sign with the squared box around it and instead adds a new command to X-Plane 11 that lets you toggle mouse yoke control on or off. When toggled on, the mouse cursor changes to a 
symbol and you can
- move the mouse right to go right,
- move the mouse left to go left,
- move the mouse toward you to go up,
- and move the mouse away from you to go down.
The absolute mouse position on the screen represents the deviation from center of the yoke, i.e.
- move the mouse to the right edge of the screen to apply full-right yoke,
- move the mouse to the left edge of the screen to apply full-left yoke,
- move the mouse to the bottom edge of the screen to apply full-up yoke,
- and move the mouse to the top edge of the screen to apply full-down yoke.
Additionally, a magenta text indicator is displayed in the upper-left corner of the screen when mouse yoke control is active.
While in mouse yoke mode, press and hold the left mouse button to switch to rudder mode. The cursor will change to a 
symbol and the magenta text indicator will read Mouse Rudder Control. Two green little bars will appear to the left and to the right of the cursor that indicate the mouse movement range for rudder deflection.
Release the left mouse button to transition back into mouse yoke mode. The rudder will then gradually return to the neutral position.
You can get the latest version here.
Simply extract the .zip archive into the plugins directory of your X Plane 11 installation, e.g. X Plane 11\Resources\plugins.
Once installed, you will find a new command for toggling mouse yoke control on or off in the Keyboard settings.
On startup, the plugin looks for a settings.ini file in its' directory, i.e. "X Plane 11\Resources\plugins\BetterMouseYoke\settings.ini". This file allows you to change some of the settings of the plugin. If the file does not exist, the plugin uses default values.
Here's an example settings.ini file with all the options the plugin allows you to configure. Lines starting with a # are comments.
[settings]
# When entering mouse yoke mode, set the mouse cursor to align with
# the current yoke deflection to avoid rapid movements.
# Set to 0 to disable.
set_pos = 1
# Change the mouse cursor symbol when in yoke or rudder mode. Set
# this to 0 to disable.
change_cursor = 1
# Defines the mouse movement range for rudder deflection in rudder mode.
# In other words, the greater the value, the greater the distance between
# the two green vertical bars on the screen.
rudder_deflection_distance = 100
# The speed (in units per second) with which the rudder returns to the
# neutral position after exiting rudder mode.
# The default value of 2.0 means that it takes the rudder half a second
# to return to neutral from full left or right deflection.
rudder_return_speed = 2.0