When your Judgement, Crusader Strike and Divine Storm spells deal a critical strike, your target will take an additional X% damage over 8 sec.
Righteous Vengeance is a Tier 10 Retribution talent which places a stacking dot on the target based on critical strike damage dealt by the Paladin's Divine Storm, Crusader Strike and Judgement Spells.
The current belief for this spell is that it will refresh the duration of the debuff, and increase the periodic damage dealt to the target based on the crit damage dealt. eg:
A Judgement crit for 1000 damage puts an 8 second dubuff of 400 damage on the target ticking for 100 damage every 2 seconds, then a Divine Storm critical strike 4 seconds later also for 1000 damage will refresh the debuff duration to 8 seconds, and will add the 400 damage to the 200 still left in the pot, and will therefore tick for 150 damage until the buff expires or another critical strike refreshes the stack. (Please confirm or dismiss this as this is my understanding)
the high damage from judgements with seal of corruption/vengeance with 5 stacks of blood corruption (which have a tendency to hit higher than any other single attack for a paladin) can cause this talent to be incredibly potent, dealing significant damage over time to bosses
quite effective in pvp against rogues and druids, who can stealth, as this will break stealth if it ticks and they take damage while hidden
Edit
Patch 3.1.0 (2009-04-14): Reduced to 3 ranks for 10/20/30%. Now triggers off of Crusader Strike. 
