I recommend that you install this mod with Long War Mod Manager (http://www.nexusmods.com/xcom/mods/620/). This mod has been tested with Long War beta 15d2 but it should work with any version of Enemy Within (Long War or vanilla XCom).

This mod displays a sight range ring around visible aliens, such as those revealed by a battle scanner. You'll be able to see how close you can move to them without being seen. No more tile counting!

Press Alt+V to toggle the sight ring on and off. If you use a game controller press the right stick in (R3) to toggle.

There are 2 version of this mod included in the zip file: 

AlienSightRangeIndicator _InactiveVisibleAliens.txt
This one draws sight rings around INACTIVE, VISIBLE aliens only (like those revealed by a battle scanner). My personal favourite.

AlienSightRangeIndicator _AllVisibleAliens.txt
This one draws sight rings around ALL VISIBLE aliens.

IMPORTANT:
===========
If you install this mod using Long War Mod Manager the following config changes will be made automatically (you won't have to do anything).

If you installed using PatcherGUI you'll have to must modify your <SteamApps>\common\XCom-Enemy-Unknown\XEW\XComGame\Config\DefaultInput.ini file as follows:

Search for the section labelled "; TACTICAL binds"

It looks like this:

;----------------------------------------------------------------------------------------
; TACTICAL binds
;----------------------------------------------------------------------------------------
.Bindings=(Name="Escape",Command="Escape_Key_Press | onrelease Escape_Key_Release | CANCELMATINEE 1", bPrimaryBinding=True)
; Confirm / Cancel binds, secondary
.Bindings=(Name="Enter",Command="SpaceBar_Key_Press | onrelease SpaceBar_Key_Release", bPrimaryBinding=True)
.Bindings=(Name="SpaceBar",Command="SpaceBar_Key_Press | onrelease SpaceBar_Key_Release", bSecondaryBinding=True)
...
.Bindings=(Name="F7", Command="slomo 1.0", Alt=True)
.Bindings=(Name="F8", Command="slomo 1.15", Alt=True)
.Bindings=(Name="F9", Command="slomo 1.5", Alt=True)
;Reset Map Rotation
.Bindings=(Name="F10", Command="ResetMapCounts", Alt=True)
;Alt-F11 doesn't work; always performs window toggle
;Hard Quit
.Bindings=(Name="F12", Command="ALT_F4_QUIT", Alt=True)

Add the following 2 lines to THE END OF this section (otherwise the bindings may not work):

.Bindings=(Name="V", Command="UIToggleAnchors", Alt=True)
.Bindings=(Name="XboxTypeS_RightThumbstick",Command="UIToggleAnchors")

It will now look like this:

;----------------------------------------------------------------------------------------
; TACTICAL binds
;----------------------------------------------------------------------------------------
.Bindings=(Name="Escape",Command="Escape_Key_Press | onrelease Escape_Key_Release | CANCELMATINEE 1", bPrimaryBinding=True)
; Confirm / Cancel binds, secondary
.Bindings=(Name="Enter",Command="SpaceBar_Key_Press | onrelease SpaceBar_Key_Release", bPrimaryBinding=True)
.Bindings=(Name="SpaceBar",Command="SpaceBar_Key_Press | onrelease SpaceBar_Key_Release", bSecondaryBinding=True)
...
.Bindings=(Name="F7", Command="slomo 1.0", Alt=True)
.Bindings=(Name="F8", Command="slomo 1.15", Alt=True)
.Bindings=(Name="F9", Command="slomo 1.5", Alt=True)
;Reset Map Rotation
.Bindings=(Name="F10", Command="ResetMapCounts", Alt=True)
;Alt-F11 doesn't work; always performs window toggle
;Hard Quit
.Bindings=(Name="F12", Command="ALT_F4_QUIT", Alt=True)
.Bindings=(Name="V", Command="UIToggleAnchors", Alt=True)
.Bindings=(Name="XboxTypeS_RightThumbstick",Command="UIToggleAnchors")

Save your changes and you're good to go.

1.0a - Initial release
1.0b - Added toggle option. Alt+V or R3 button on game controller.
1.0c - Two versions of the mod are now included in the zip file. One for inactive, visible aliens, and the other for all aliens. Sight rings should now always remain hidden until it's the player's turn.
1.0e - "All" version was displaying rings for dead aliens - fixed.
1.0f - Sight rings were not being hidden during Exalt's turn
1.0g - Sight rings were drawn on stealthed and stunned enemies. Should be ok now
1.0h - Updated mod file to support Long War Mod Manager auto config