Here are a few that I use at the moment on my target unitframe:
Name - Displays the target name, their status when you mouseover the frame and displays whether or not the target's a boss (while we're on this topic, is there any better dogtag method of checking if the target's a boss?)
[Name] [IsMouseOver ? AFKDND:Angle] [Classification:Contains(Boss) ? Text(Boss):Paren:Gray]
Spec - Displays the target's talents.
[IsPlayer ? IsEnemy ? TalentTree] [IsPlayer ? IsEnemy ? TalentSpec:Prepend( - )]
Debuffs (I have this tag showing in a blank bar on Pitbull) - If the target's in combat, lists what tank damage mitigation and melee damage increasing debuffs the target has currently, shows -TC when there's no thunderclap present, -CoR when there's no Curse of Recklessness present, etc. (Supports TC, DS, CoR, FF, FF (Feral) and SA. I'm also currently thinking of adding a check if the target's a boss or not since some of this data is kinda useless on trash mobs)
[Outline][CanAttack ? InCombat ? [HasAura(Thunder Clap) ? Text() ! Text(-TC):Red] ] [CanAttack ? [InCombat ? [HasAura(Demoralizing Shout) ? Text() ! Text(-DS):Red] ] ] [CanAttack ? [InCombat ? [HasAura(Blood Frenzy) ? Text() ! Text(-BF):Red] ] ] [CanAttack ? [InCombat ? [HasAura(Faerie Fire) | HasAura(Faerie Fire (Feral)) ? Text() ! Text(-FF):Red] ] ] [CanAttack ? [InCombat ? [HasAura(Curse of Recklessness) ? Text() ! Text(-CoR):Red] ] ] [CanAttack ? [InCombat ? [HasAura(Sunder Armor) ? Text() ! Text(-SA):Red] ] ]