Passworded levels:
You will always find the username and password either at the same time, or in the same way.

Level 1: You know how to open doors. Right?

Level 2: It's sealed tight, you need to find another way around. There's nothing to click.
A link isn't the only way to change what page your on. You're on Level 2. How can you get to Level 3? just try it!

Level 3: Something is missing. Two things actually, since you need a username AND password.
Think about it. What isn't there, that should be?

Level 4: How do you defeat this creature? The source code might be able
to help. Dunno what source code is? You do know you're on the internet. Right?

Level 5: The page title is a big hint. See if you can find something. Can't use it as it is?
again. You're on the internet. Find out how to use it!

Level 6: The URL can help you here. There's more than one way to do what it tells you. Do them all untill you see it.

That's it! No more hints after level 6, you're on your own from here!