That Just Happened is a simple but very useful WoW addon that allows you announce (locally, or the party/raid) when certain events happen, like Guardian Spirit on someone, or when a Polymorph breaks.

Below are the rules I currently use:

SPELL_AURA_APPLIED;;;Hand of Salvation;;RAID;Hand of Salvation on *target (*source)
SPELL_AURA_APPLIED;;;Hand of Sacrifice;;RW;Hand of Sacrifice on *target (*source)
SPELL_AURA_APPLIED;;;Guardian Spirit;;RW;Guardian Spirit on *target (*source)
SPELL_AURA_APPLIED;;;Shield Wall;;RW;Shield Wall on *source
SPELL_AURA_APPLIED;;;Divine Protection;;RW;Divine Protection on *source
SPELL_INTERRUPT;;;;;RAID;*source interrupted *target
SPELL_AURA_APPLIED;;;Pain Suppression;;RW;Pain Suppression on *target (*source)
SPELL_CAST_SUCCESS;;;Divine Sacrifice;;RW;Divine Sacrifice (*source)
SPELL_AURA_APPLIED;;;Hand of Protection;;RW;Hand of Protection on *target (*source)

(We’ve not had a DK tank for a long time, so no CDs listed for them)

Maintankadin has a great overview with many more examples