Want to see how well you know HTML5? or learn from others' collaborative problem-solving?
Here are two quizzes (actually, one's a series): the first tests your knowledge, the second your understanding.
The basic ('stormtrooper level') quiz at Nettuts+ simply asks a few questions whose answers could be pulled straight from the spec, concentrating on 'plain facts about what's new in HTML5'. This will be useful to remind you of what new things you can do in HTML5, and what old things you can't.
Some of the questions might seem a little silly ('which of the following doctypes was introduced by HTML5?' -- once you start writing HTML5 pages, you'll have used this too many times to count), and some are more political than practical at this point ('in which year is the HTML5 specification supposed to be complete and finalized?' -- but 'complete and finalized' from W3C always occurs long after major use). The major utility of this quiz is: keeping HTML5 innovations clear in your head.
Take this quiz if you're thinking about transitioning into HTML5 development.
Contrariwise, the HTML5 Simplequiz series at html5doctor.com doesn't offer pat, trivia-style right/wrong answers. Instead, Bruce Lawson presents a real-world problem with several possible solutions, then asks programmers to choose one solution and explain why. The value of this exercise, Bruce observed, lies not so much in the answer as in the discussion: others' thoughts and justifications will help you understand how programmers think about doing actual development in HTML5.
Think of these quizzes as the happy developers' cave, wherein insights and suggestions fly freely and all minds synergistically grow. Also, the questions are considerably more complex than the Nettuts+ quiz's, and apply a number of more esoteric (and less sexy) features of HTML5 (such as <figcaption>).
Work through this series if you're already familiar with the spec and want to get a feel for how others develop in HTML5.