Per request, I wanted to provide a brief post on raiding addons for dps DKs.
This is not intended to be an in-depth review of addons, just a brief mention of the addons I use in raids and what kinds of addons you should be using. Maybe in the future I'll do more in depth reviews of specific addon categories.
1. Raid Warnings/Events: DBM (Deadly Boss Mods) is the industry standard, but there are a few others out there that work ok. Mostly, it makes a huge difference to get warnings when certain events occur. Examples include standing in fire on Koralon, when to Mind Freeze on Val'Kyr Twins, when you have Light Bomb on you on XT Deconstructor, and on and on and on. Useful in EVERY fight.
2. Proc Alerts: Most of the mechanics to maximize your output as a DK require you to time cooldown use when they will have the greatest effect; particularly when you have the most procs up. For example, you want to pop your ghoul and dancing rune weapon when your Darkmoon: Greatness, Sigil of Virulence, Rune of the Fallen Crusader, and so on, are all active. You also need to know when your Horn of Winter drops, When Rime procs, or when Bone Shield is ready. Mik's Scrolling Battle Text is my favorite; it's pretty, very customizable, and has a nice format. Main thing I do is have both a visual alert AND a sound that will tell me what I need to know about my procs/buffs.
3. Rune Watch: I'm a 'thowback.' I like Rune Hero. Main thing is, though, that you need to know which runes are ready to be used, which ones are almost off cooldown, and how much runic power you have. You have many options - try a few out and see what you like best.
4. Communication: Vent. Not really an addon, just saying. Get it.
5. Threat Meter: You need to know if you're about to pull threat (and wipe the raid). If you have the talent butchery in the blood tree, this is unlikely, but it's still helpful. Omen is by far the most popular threat meter.
6. Others: For the most part, any other addons are for personal preference. @Ayeba: the addon in my screenies that puts a semi-circular health/runic power bar next to my toon and the enemy's health/mana (etc) on the other side is called MetaHud. I'm a long time fan: it definitely helps me have that useful information right there if I need to use cooldowns or a healstone/health pot. I became addicted to it as a rogue when pooling energy, but it's nice for every toon I use. Others I use include:
-Grid. You don't need this as a dps DK, but I do like to see who's alive during a fight, or be able to quickly find someone to target their target etc. It's essential for healers/raid leaders, not so much for dps DKs.
-OneBag. I love bag mods that open all your bags at once.
-Postal. Makes mailing and opening batches of mail much easier, as well as keeping lists of who you've mailed.
-Altoholic. I make notes of people I don't like (stay away) and those that I do (good dps, tank, heals, etc).
-Carbonite. Overall, an AMAZING addon that I recently jumped on the bandwadon for. Better than Questhelper for leveling or doing dailies. Better map functions. Better gathering addon. AND it takes up much less ram.
-Informant. Any addon that lets you compare your gear to the loot that dropped is nice.
I use a few more but I'm going from memory here. :)
-UI mods. I don't use one, but there are many popular and beautiful ones out there. XPerl... very good.
Any one else want to chip in? Any addons you find indispensable? Useful?
By the way, where do you get these addons? For the most part, I use Curse. You can get many of these direct from the developers as well, but right now the only specific site I go to is for Carbonite. Another very popular addon site (I don't use very often myself) is Ace. It has a lot of developers out there and sometimes has updated addons before Curse does.
Happy addon hunting!
EDIT 16 Oct: I just realized I didn't mention Recount. I do use the meters all the time. Once should first and foremost use meters to measure your own improvements and look to how you can improve yourself. They can be helpful, however, to let you know if you're doing something right, or wrong. A fellow DK who is doing 1k more dps than me? Hmmm, what are they doing? Rotation, gearing, gems, spec, etc. Meters can provide a very helpful resource. WMO Online also provides very detailed information and may be posted by your guild to assist in analysing your performance.