Knocking out guards with objects.
I found that this works on the level that you have no epuipment(don't remember the name), find a guard that is unaware of your presence and pick up an object. Aim the object at their head and throw it at them, if they do not know that you are there they will be knocked out, I'm not sure if this works on aware guards, I am still testing that, I will post an update when I figure it out. Suggested objects are either cans or bottles, other objects may work too.
Added by: Zach22 Jan 1st 2010, ID#7924