Overview: A simple mod - it tracks every unit you deal with during a fight, and tracks what buffs are gained/lost by that unit, and stores that information, so you can see it later.
Reasoning: Don't you hate it when you're in the middle of a boss fight, and you're like "what the hell is that debuff?" and you (if you're me) stop generating threat or looking at your hit points and go mouse over the debuff. Then, of course, the healer cleanses you, you never see the debuff, you're standing in lava, the boss just swiped the raid, and you lost threat on an add. And all you can say in vent is "I wanted to know what the debuff said..." And you still. Don't. Know.
Enter xAuraTracker. Every buff or debuff that you, your target, focus, party, raid, or the boss/arena team you are fighting against gains or loses will be tracked, as much information as possible about it stored, and you can review it after.
When you first install, you will see a lot of buffs and debuffs on the list, and these will just be your common procs/group buffs/etc. This is what the "Ignore" feature is for. There's two types of ignore - "ignore all" (never shows that buff/debuff again, no matter who cast it) - and "ignore from source," which will not tell you about that debuff when mobs with that name cast it (if we know the source), but will list it again if a new mob casts it. (This is a great chance to actually read the text on some of those proc buffs you never realized you had!)