The amazing quester plugin from ragan262

Quester is highly customizable plugin handling all kinds of quests. From simple “gather to ret reward” quests, to complicated forked story-telling quests. What is even better, everything is customizable on-the-fly directly in game ! No more restarting/reloading your server every time to change something. Localizable messages, plenty of different, highly-customizable objectives, conditions, events and flags. NPC and sign support. All this together creates almost unlimited possibilities.

http://dev.bukkit.org/bukkit-mods/quester/