Diminish attaches icons to unit frames or nameplates, displaying time left until a crowd control diminishing returns category expires for that unit. As well as how far the category is diminished. (Green border = 50% duration, yellow = 25%, red = 0%)
Diminish can track DRs on both mobs and players if you enable PvE mode in the options.
Note: This addon is no longer being updated. It should still work for now unless there's some drastic client changes. If you liked it over the years, consider giving feedback to Blizzard about adding DR tracking built-in to the game. After all, DRs are a 16+ year old CORE pvp game mechanic and it doesn't even have an addon API.... everything is aura scanning guesswork.
Type /diminish to open the options panel. You may also skin the icons using Masque.
DR icons for each unit frame is separately configurable. Certain unit frames are default disabled for tracking or only enabled for certain zones.