Most people know about about Godwin's Law. In short (quoting Wikipedia), this adage states: "As an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches 1."
Here are a few other internet forum laws with particular relevance to the New Zealand scene.
Slater's Law: this is similar to Godwin's law. but swap out "Hitler" for "Cameron Slater" and "Nazis" for "Whaleoil". This law can be seen in practice on various left blogs.
Trotter's Law: Whatever Chris Trotter predicts about the left, the exact opposite will eventuate.
Mclauchlan's Law: Whatever the Dim Post predicts about the left will almost always eventuate.
The Red Alert Own Goal Law: All the good work put in by hard-working MPs on Labour's blogsite will be undone in an instant by one person's intemperate post.
Edwards' Law: As you get older and have more time to reflect on things you will use your blog to turn on everything and everyone you once loved.
Farrar's Law: The more time you spend reading the comments the more your health will suffer.
The Law of Common Sense: As a blog discussion grows longer, the probability that Pete George will turn up approaches 1.
Brown's Law: Every blog thread will descend into a discussion about coffee.
Edgeler's Law: Graeme knows more than you do about everything.
I'll keep adding more as I/you think of them.
Have I missed any?