Since early 5.1 and the advent of the Festerblight spec, many Death Knights have become familiar with DKDots. Written by Simu of EU-Grim Batol (who based it off AffDots), the addon was a must have for maximising disease damage- particularly for Unholy Death Knights. It tracks Crit, Mastery (for Unholy/Frost), Attack Power and % Power Gains (i.e. things like Tricks of the Trade or Primal Nutriment on Ji-Kun) to inform players when the best time to (re)apply or extend diseases is.
Unfortunately, 5.2 saw the addon begin to malfunction somewhat- randomly crashing or not loading in combat. Simu has quit the game, which is why there haven't been any updates to it as of yet.
Fortunately, #Acherus's own Kuren (Kilrogg-US) has resurrected the project with a new addon written from scratch: DKDiseases. The addon can be found available for download here: http://www.curse.com/addons/wow/dkdiseases
In terms of both functionality and aesthetics, it operates identically to DKDots. Testing thus far has simply shown it to be more stable and less prone to crashing in 5.2. If you have been experiencing issues with DKDots and are looking for a replacement, then this is the addon for you.
If you're a newcomer and have never heard of either addon, now is a good time to try it out. I personally consider such trackers to be absolutely necessary for anyone looking to play Festerblight correctly.
Please take the time to thank Kuren for his hard work- the community owes him one.