I've been spending way too much time with the Harry Potter Knit & Crochet House Cup group on Ravelry, mainly because they've had awesome Quidditch projects this term. For example, there is the Gryffindor bookscarf, shown with The Poisoner's Handbook by Deborah Blum, a very good book about the development of forensic science in the early 20th century. The pattern, which uses DMC Perle Cotton #5 and tiny 2mm needles, can be found here.
Then, there is the Hedwig Illusion Dishcloth pattern from Freshisle Fibers. When you look at the dishcloth one way, it's a plain old boring striped dishcloth, but look at it from an angle, and the surprise is revealed!
Finally, I have a complete set of Pocket Potter Puppet Pals, based on the famous Mysterious Ticking Noise video. This pattern is also free from Inner Child Crochet. I really, really hated doing all the fiddly seaming, and I was going to abandon the project after completing Snape. However, Kate begged me to complete the set, and once I adopted an assembly-line approach, it wasn't such a struggle.