CountEntities lets you count the entities in your chunk in a very simple way - with just one command.
It shows up all the entities and splits them into a few categories: Hostiles, Neutrals, Passives, Players and Bosses. A list can be found here: http://minecraft.gamepedia.com/Mobs#List_of_mobs
Works fine with Bukkit/Spigot 1.7 and 1.8 and should also with older versions.
Simply drag and drop the downloaded CountEntities.jar into your plugins folder and reload/restart your server.