Besides Wikipedia, the Wikimedia Foundation operates 14 other projects with the same goal as Wikipedia—to make knowledge free and widely available—but focusing on different types of knowledge. Also like Wikipedia, all are published using wiki software, created and run by volunteers, and published using a free license.
Some of these, you might already know about: Wikimedia Commons is where most of the images used on Wikipedia are stored; it also hosts audio and video, but mostly images. Wiktionary is the open content dictionary (alas, not the wiki version of Pictionary).
Some you probably have not: most important in recent years is WikiData, a database that also provides information to Wikipedia and for research purposes. On Wikibooks you can find the full text of thousands of public domain texts.
But the most popular by far is the one and only Wikipedia.