Ev Williams co-created Twitter way back in 2006—years before you were using it. He's kept a firm grasp on the company since then, and according to Bloomberg News sources, still holds 15% of Twitter. That means the biggest payout on IPO day.
Williams is already rich: somewhere in the vicinity of $200 million rich, depending on sources (earlier in his career he created Blogger!). But he'll add $1.5 billion to that pile when his bird baby goes public, the largest out of any equity holder. That's also assuming a $10.5 billion valuation of the company—the real deal could be higher. Or, as Mark Zuckerberg will tell you, perhaps lower.
Either way, you are looking at what will be the happiest face on that sacred day of microblogging monetization. If he's smiling up there in that photo, just think of what one and a half billion bucks will do.