There's no doubt that Wikipedia is a giant store of information. And yes, they have an API, but the data isn't exactly structured. But that didn't stop Jim Blackler, who created Quizipedia, a fun game based off of Wikipedia entries.
The concept is fairly simple: guess which of ten words fits into a blank space in a sentence. As you guess correctly, your choices narrow, but you only have one minute to get them all.
The part we see is fairly simple. It just needs to grab ten potential answers from the database. The more complex part is how Blackler went about finding the the articles to use, which he explains in a post about how he made it. For example, he used links within Wikipedia to determine whether an article would be too obscure.
It's fun to see these sorts of mashups. They build upon data that's already out there for other purposes. If something this interesting can be made from Wikipedia's unstructured data, imagine what is possible with something like Factual's open data platform or similar public datastores.
Hat tip: Andy Baio