Links

Omen Required for: heroics/raids - Calculates amount of threat generated by your spells in comparison with those in your party that also have this addon installed.
Recount - Calculates your damage or healing. Options include overall damage, raid damage graph as well as other hotness.
Healbot Required for: heroics/raids - Required Healing addon for raiding. Shows outgoing heals to reduce or correctly plan for overhealing.
BigWigs - Boss addon to warn the player of impending doom. Must haves for high end raiding or anyone wanting to stay alive. (Link on second page)