Very reliable, often updated. Easy to customize:
SPTimers.
Displays DoT ticks, which some people find crucial (like me) and not every DoT timer does it.
It also shows all units with our DoTs, not only target and focus, and this is probably unique for this addon.
Contrary to what this addon is assigned to (Class specific: priest), it will work with any class. I've used it for my spriest, warlock and balance druid.