Complex sentence: The Wikimedia Foundation is a non-profit and charitable organization headquartered in San Francisco, California that operates several wikis; with its most notable being Wikipedia, which ranks in the top-ten most-visited websites worldwide.
Split into simpler sentences: The Wikimedia Foundation is a non-profit and charitable organization headquartered in San Francisco, California that operates several wikis. The foundation is mostly known for operating Wikipedia, an Internet encyclopedia which ranks in the top-ten most-visited websites worldwide.