Doppelganger is a Bukkit plugin that provides an alternative game mechanic for acquiring player heads that might be useful on PvE (Player vs Environment) servers. In order to acquire a specific player's head, one must summon that player's doppelganger in a manner similar to summoning a snow or iron golem. To do so, use an anvil to name a "trigger block", e.g. a pumpkin, the same as the desired player, then place that on top of a stack of golem blocks, e.g. two diamond blocks. The type of the trigger block and the type and shape of the blocks you stack it on are all configurable.
You must then kill the summoned mob in order to get the head.