The Language Construction Kit
zompist.comI bought the paper book and was a bit disappointed by it. The material is fairly basic, so that you don't learn much, and the quality of the typesetting is quite poor... :/ (Maybe the first criticism doesn't apply to the "advanced" sequel, though.)
Upvoted for generously delivering information that you think would be helpful and that I couldn't have known without buying the book myself.
Whoah, I've enjoyed this "webchapter" for 12 or so years! It's how I got interested in linguistics.
Be sure to click on the "What a geek you must be. Get a life!" link.
I was hoping this was like OSKit for programming languages. Of course, I suppose that's called Parrot.
If you are newly interested in conlangs then check out ithkuil. Very advanced in a way and quite interesting.