DebuffDock

DebuffDock puts the debuffs you care about in one tidy, movable bar. When your target is missing one of your debuffs, its icon lights up so you know exactly what to recast. Click the icon, or press its hotkey, to cast it. While a debuff is running, the dock shows how long it has left.
Features
1 to 12 slots. Drag any spell from your spellbook onto an empty slot. Corruption and Immolate are set up for you to start with, and any spell can go in any slot.
Clickable and bindable. Click an icon to cast it, or give each slot its own hotkey (Shift, Ctrl and Alt combinations work).
Timers show how long each debuff has left on your target.
Shows up when it matters. By default it appears only when your target is missing a debuff, and during combat. You can also keep it on screen all the time.
Fully movable. Drag it anywhere, resize it with the corner grip, change the gap between icons, and Ctrl+click two icons to swap them.
Your text, your way. Change the text under the icons, for example "Recast %s!", or hide it.
Optional transparent background and spell tooltips.
Everything is saved between sessions.
How to use
Type /dd to open the settings window, or use the minimap button or Options → AddOns → DebuffDock. Type /dd help for the full command list.
Because of game rules, slots, hotkeys and layout can't be changed during combat.
Disclaimer
DebuffDock is one of a few addons I came up with to make quality of life easier for myself during the WoW Forever beta test. The ideas are my own, and I did not search for, look at, or use any other author's addons, code or artwork before making them. Any resemblance to existing addons is coincidental. Made for WoW Forever. Released under the MIT license.