Just hit 80 and not sure what addons you need to have on your Death Knight to be the most effective tank/DPSer when you begin raiding? Fret not! Here I've compiled a list of addons that you should have, might want, and ones that are just plain cool. Let's jump in, shall we?

DeathKnight.Info Runes
Must Have

DeathKnight.Info Runes is a Rune Timer, which you can find all over the place. The difference here is that it also tracks your diseases. It even shows you if Pestilence is up on nearby targets or not. The addon has a nice looking Rune Sword as a tracker (see image above) and it's fully sizable, rotatable, and movable. Definitely a must have.

Face Smasher
Must Have (maybe)

The reason I say maybe is because this is a great addon, but in all honestly it takes the fun out of it, for me anyway. What Face Smasher does is analyzes your runes, and runic power. It then decides for you what you should cast, and immediately puts it on the screen for you to click. This is an incredibly useful addon but makes me feel like I'm beginning to let the game do my job for me. If you are trying to get the most out of your character then I reccomend this.

Quiet that Horse!
Just plain cool

For those of you that still ride your Deathcharger, this ones for you! This changes the annoying sound of your mount into a 'subdued, but still atmospheric roll of muted thunder' I believe he called it!

Nonce
Just plain cool

Though it can also be helpful as well! Nonce displays a colored aura around your screen to show you what presence you are in (Frost, Blood, or Unholy) The aura looks pretty spiffy, but can get annoying sometimes. It does help to remember to switch auras though! I can't count how many times if needed to OT something so I switch into Frost Presence, and then I forget to switch back to Blood/Unholy the rest of the instance. Cool, and possibly useful mod to have.

Death Knight Alert
Must Have

Death Knight Alert shows a flash and/or a sound effect every time a talent procs a buff that has a limited timer on it. This is a must have if you want to get the most out of your build.

These are all the Death Knight specific addons that I reccomend, however, these are not the only ones you should have. Raiding, Tanking, Threat Meters, Damage meters. There are many many other addons out there that will help you and your group. For more addons head on over to Curse.com