Aggro Yards is an addon which displays the approximate aggro radius of your target. This addon does only basic "pen-and-paper" calculations to determine the radius based on target level and player level and other minor factors.
To gauge the distance you must use your various spells as reference, because unfortunately, it is currently impossible to determine your distance to a mob. In most cases, the radius displayed is a little larger so you get a bit of a margin when estimating distances.
Estimates current target's aggro radius
Only works on targets that will attack you
Frame hides when you enter combat
If you lose the frame, use /aggroreset to set it to an on-screen position
Since I have made this addon I have used it all the time. It is easily integrated into any UI and very useful.