With a title like Crusader Kings II we could not have not included the crusades, and, in what must class as a piece of unintentional irony by Doomdark, I have been delegated to write it.
First off, for those of you who really are getting immersed into the world of Crusader Kings II and simply cannot wait for the Pope to call a crusade there is nothing stopping you going off on a solo tour. You always have a CB against religious enemies, so if you have the will we have given you the means. The CB targets a De Jure Duchy of a religious enemy, and on success you acquire all the holdings inside the Duchy, leaving you to just decide who will be the new bishops and mayors.
The Crusades themselves are in fact not just crusades; any independent religious head may call for a holy war against religious enemies. The religious heads also have certain preferred targets, just as the Pope would love you to get Rome back (if lost), the Muslim Caliphs want the faithful to recover Mecca and Medina (so expect an interesting stay if you do decide to visit). Success in conquering the Crusade target grants the conqueror bonuses and prestige and piety.
As a little aside, since I know it is probably going to be one of the first questions you are going to ask, this is all very moddable. A holy war is scripted as an EU3 style mission that is open to all characters of that religion. The logic that decides when it is called and what province is the target is stored inside this script allows those of you who are of the modding persuasion to play around with it to your hearts content.
We also have events tied into the crusades. These include events that supply a bonus to characters who arrive at the crusade target and participate in its capture. Vassals may even be disappointed if you do not participate in the Crusade and the Pope will also badger you to take part in it. He may even offer to lift an excommunication to persuade you to go along.
We also have holy orders (again in a nice script file for those of you who want to add more), these act as very powerful mercenary regiments. However, they have two twists: they will not fight people of the same religion and they cost piety instead of cash to hire. Bringing these guys along on a crusade can be very helpful. In addition, they are classed as Dukes for game purposes. So they can be granted titles, create vassals and be vassalised. In our set-up files the Hospitalers and the Templars are scripted as Vassals of the Kingdom of Jerusalem (which means they are no longer for hire) and they form a powerful shock force of the Kingdom. The famous Krak de Chevaliers is a separate castle holding held by the Hospitalers.
Once Holy Orders gain land they also gain an AI and diplomacy options, meaning that if you were to, say, grant the Teutonic Knights a few holdings in Prussia they might just get it into themselves to expand even more. There are events where the Holy Orders will ask for castles, and as these formations are militarily powerful, granting them castles will form a powerful bulwark against those who would seek to deny you your rightful holdings.
All in all Crusades have been given a nice little make over in CK2 that makes them much more interesting and dynamic.
Grand Master of the Holy Game Design Order.