What's the difference between yet another and another?
You get the first one (of whatever it is that is under discussion), then you get another of them, and then you get yet another of them.
So, it could be argued that 'yet another' implies at least the third.
It could also mean that something is happening tediously often:
First one interruption, then another, and another, and then yet another. Aaargh! How am I meant to get any work done?
"Yet another" isn't just more emphatic. It also changes the focus.
Consider the following sentences.
Another question was asked on the subject.
Yet another question was asked on the subject.
In the first sentence, the focus may be on the question or the subject. In the second sentence, the focus is on the fact that there are so many questions.
As explained by @JSBangs and @Jonathan, "yet another" is used as an emphases on the fact that there were many more before.
Often it can be used as an expression of the exasperation to see something for the penultimate times
Yet another internet meme...
In fact, it is also used at the beginning of acronyms designated something that was made so much time that finding an original name is quite difficult.