Obviate allows you to see what abilities an NPC has before combat.

Information is shown in the default chat frame. The ability links work (you can mouseover for details, shift-click to quote them into an chat box). The built-in database contains information on over 2,900 NPC's including most instances from Wrath of the Lich King and Burning Crusade and several from Cataclysm. Automatic data collection means that as soon as unknown NPCs use abilities they become known to the addon. Uses an embedded copy of Ace3.

Comparison with similar addons
Version History

Commands:
/obv8 - prints a list of known abilities for your selected target
/obv8 say - announce target NPC's abilities to chat
/obv8 help
/obv8 config
/obv8 active - opens active report window
/obv8 setnote - view/set npc notes

Completed Features

Can make reports to chat ("say" only atm)
NPC's with known summoned creatures will list their offsprings' abilities too (recursively) (note: summoned mobs can only be added to the DB manually)
Automatic data collection from combat log (can be disabled)
Settings are saved between sessions on an account wide basis
Only reports on spells seen at the current instance difficulty (ie, normal / heroic, 10 / 25 raids)
Support for "vehicle" bosses
Ignores blacklisted spells (eg. Dazed, Haunt)
Heal, interrupt and summon abilities are clearly marked
Active window allows for reports on multiple NPCs (experimental)
NPC notes can be added in-game.

Upcoming Features
Upcoming features are listed at the CurseForge ticket tracker.

NPCs will be marked as prone / immune to various CC abilities (Polymorph, Freezing Trap, Slow, Sleep, Cyclone, Roots, Sap, Kidney Shot, Blind, Gouge, Banish, Enslave, Fear, Death Coil, Trap, Hex, Shackle, Mind Control, Turn Evil, Death Grip, Dark Command, Scare Beast, Infected Wounds, Curse of Tongues, Curse of Exhaustion, Growl, Taunt, Righteous Defense, Maim, Bash, Repentance, Psychic Horror & Psychic Scream)

Known Limitations / Bugs
Known bugs are listed at the CurseForge ticket tracker, the most important ones are:

Newly tamed hunter pets are still classed as NPC's until dismissed. NPCs are credited with hunter pet abilities
This addon has only been tested on English clients running on US servers. There's no localisation at all in the addon at the moment. All spell and npc tracking is done by ID's which should be locale independent.