Need To Know is a nice small and uncomplicated addon that lets you have up to 4 groups of 5 bars wherever you want on screen, to show spell cooldowns, DoTs/HoTs, buffs/debuffs and totems.
The bars NTK shows always appear in the same place when active, and disappear otherwise. Taking the above screenshot as an example, once Anti-Magic Shell comes off cooldown (thus causing the bar to disappear), the Unbreakable Armor and Icebound Fortitude bars would not move downwards to fill the empty space. This helps to maintain a consistent mental map of where things are.
Bars are coloured manually - the default is medium grey. Each bar can optionally show an icon for its related spell, buff etc.
The Frost Fever and Blood Plague bars are tracking their respective DoTs on my current target. When set to Buff or Debuff mode, the bars can be set to track either the player, your target, your pet, and so on.
Another handy feature that I only read about as I was writing this post is that you can set bars to track multiple buffs/debuffs: it shows the remaining time for the shortest, then when that expires moves to the next one. Another possible use for this, although I've not tried it, is loading a bar with all important vehicle abilities you're going to encounter, so for example in Oculus you can have a single bar for tracking Leeching Poison stacks (green drake), Shock Charge (amber drake) or Evasion Charge (red drake). You could even further load this bar with other stuff like pyrite stacks for the Flame Leviathan encounter in Ulduar.
That's more or less it really: it's a lightweight addon that does one particular thing, and does it well.
Here's a brief explanation of things in the screenshot that aren't related to NTK:
- the health bars (and target buffs/debuff icons) are by Pitbull 3.0
- the RS, GCD and Mind Freeze "indicators" are really just 3 buttons on a small Bartender4 bar that isn't clickable
- the "GCD indicator" is actually just a button for a stamina scroll
- I have Rune Strike macro'd to all my main attacks, so most of the time I don't need to manually activate it. However, if all runes are on cooldown and I only have 20 RP, the Rune Strike indicator lets me know when the ability is ready so I can use it manually
- the Mind Freeze indicator serves as both a range check, a cooldown monitor and an availability check, all in one handy icon
- the runes display and runic power bar are by Magic Runes
- the numbers in the top-right are MikScrollingBattleText. I should really make the crit display a bit smaller