With the exception of Southsun Cove, every zone has at least one Meta Event chain, each of which culminates in a "boss fight". Since it's an event chain, you'd have to turn up at just the right time or participate in the whole series of events preceding the boss fight to actually face the boss and get the reward chest. (Speaking of the reward chest, note that there has been a change recently to give you a guaranteed Rare or better item per account per day from boss chests, or rather, from an additional chest - see the official announcement.)
For a complete list of meta events, have a look at the wiki page, which has been updated since DavidYell's answer was posted.
Quite often, people will announce meta events on map chat (e.g. "the maw is up!"). Alternatively, there is a very nice tool to let you find out when meta events are due at Guild Wars Temple's Dragon Timer (you may wish to change the server of course).