Jimmy Wales is the most famous person associated with Wikipedia, and it's not even close. He was in charge of the company where Wikipedia got its start before he spun it off as a standalone nonprofit.
Even though Jimmy (or Jimbo, as he is sometimes known) hasn't been in charge since near the very beginning, he holds a seat on the nonprofit's board of trustees, and his opinions on Wikipedia matters carry more weight than any other person's.
But this doesn't mean his word is law, and veteran editors often show up on his user page to argue with him over matters of policy, or ask him to take their side. Most of the time, like a half-interested god-king, he listens but says little.
These days he lives in London, and is most visible to Wikipedia and its readers on the occasional fundraising banner. For many, he's simply Wikipedia's mascot.