Warrior(tank):
Satrina's
CombatMonitor (post-combat incoming damage breakdown, to see if my mitigation is suffering someplace in particular),
Grayhoof's
Scrolling Combat Text , Barryj
SpellAlertSCT (Blizz can imitate but can't replicate the original),
SorrenTimers ,
Gello's
ItemRack (fast gear switching),
Cirk's
Targetz (who is targetting what indicator on tooltip),
Cirk's
Ammocheck (you can't pull without ammo),
Iriel's
RedRange and
WatchCombat (toc update needed) (get early warning when a party member stumbles on a creep),
Ashenstaff
LifeAlert (party getting damaged notification).
Not listing general use mods like sRaidFrames, Ora2, BigWigs although I use those too but not related to the class specifically.
Another personal favorite is WankooMP3 (also not class related
)
Edit: Links added