The entrepreneurs on the following list have several things in common: they're rich (mostly millionaires, and a few billionaires), they're powerful (we're talking the heads of some Fortune 500 companies, here), and they can't sit still for a second.

These serial entrepreneurs have had some big hits in their lifetimes, and now, they're hoping their luck (coupled with some hard work and strategic cash infusions) will strike twice. This year saw some of the biggest names in business--Jeff Bezos, Pierre Omidyar, Sheryl Sandberg, and more--take on entirely new challenges and launch new ventures that would, once again, test their chops at running a business

Here's a look at their second acts:

Chad Hurley and Steve Chen: From YouTube to MixBit

After selling their year-old startup YouTube to Google for $1.65 billion in 2006, Chad Hurley and Steve Chen joined forces again this year to launch the video editing tool MixBit. MixBit allows users to shoot 16-second video clips and edit them together into an hour-long finished product. They can also collaborate with other MixBit users and edit other people's videos. "We're trying to build tools that are simple and intuitive, but powerful enough to help people tell those stories, and more importantly, to tell those stories with other people," Hurley told Inc. in August.

So far, MixBit's big claim to fame is the fact that Hurley secretly videotaped Kanye West and Kim Kardashian's engagement and posted the video on the site (which got him in some legal trouble with the bride and groom to be).

Jeff Raider: From Warby Parker to Harry's

Jeff Raider was part of the four-man team of University of Pennsylvania buddies that launched Warby Parker, the buy-one-give-one eyeglass company, in 2010. Warby Parker proved that, with exceptional customer service and a free home try-on option available to them, customers actually would shop for eyeglasses online.

This year, Raider decided to apply that theory to the wild world of men's grooming products, launching Harry's, an online retailer of shaving products for men. Then, this October, Harry's announced it would be opening an actual barber shop called The Corner Shop in New York City. As Raider told Inc.'s Jill Krasny at the time, he wanted to strengthen the brand's connection to its customers.

"There's always context that you miss by not being able to touch and feel things," he said. And though Harry's hasn't disclosed revenue, Raider told Krasny the company "is ahead of where Warby Parker was at this time." Considering Warby Parker's lightning-fast growth, so far, so good.

Jeffrey Hollender: From Seventh Generation to Sustain condoms

It wasn't the easiest parting of ways between Jeffrey Hollender and his first company, Seventh Generation. In 2010, after Hollender stepped down as CEO of the company he co-founded in 1988, Seventh Generation fired Hollender and removed him from the company.

Hollender was down, but not out. Now, he's joined forces with his 26-year-old daughter Meika to launch a new company called Sustain, which will sell condoms, of all things. But not just any condoms. A social entrepreneur at heart, these condoms will be toxin-free, fair trade, marketed to women, and 10 percent of profits will benefit reproductive health initiatives for low-income women in the United States.

As Hollender recently told me when he stopped by Inc.'s headquarters, "Rather than creating products that are less bad, we're thinking about how to create a model for business that does net good."

Pierre Omidyar: From eBay to "NewCo"

In the years since he left eBay, Pierre Omidyar, one of the richest men in the world, has dedicated his life to philanthropy both in the for-profit and non-profit space. Most of his work has fallen under the umbrella of his parent company Omidyar Group, but this year, Omidyar announced he was launching something completely new. For now, he's calling the venture, simply NewCo, a new media venture that will be headed up by Glenn Greenwald, the journalist who first brought the NSA's mass surveillance program to light in The Guardian this summer.

Omidyar is said to be coughing up as much as $250 million on the venture, which he hopes will "convert mainstream readers into engaged citizens," as he wrote on his blog in October. For now, details on the new venture--aside from its emphasis on investigative journalism--have yet to be disclosed.

Michael Birch: From Bebo to, uh, Bebo

Michael Birch sold the social networking site Bebo to AOL for $850 million back in 2008. Two years later, having failed to compete with Facebook, AOL reportedly sold Bebo for a measly $10 million to a hedge fund called Criterion Capital Partners, which subsequently drove the company into bankruptcy in May.

Now, Birch is trying to revive the company he started, announcing this July that he bought Bebo back. Bringing the company back from the brink is going to be an uphill battle, Birch admits, but he told me this summer the experiment was worth the (admittedly small) financial risk. "It would be lovely for it to actually make a comeback," he said at the time. "It'd be a proud moment, but in the grand scheme, it's also a relatively small gamble on our behalf."

If this genius marketing campaign is any indication, though, the future for Bebo could be bright.

Sheryl Sandberg: From Facebook to 'Lean In'

OK, so technically Sandberg did keep her day job, but 2013 will henceforth be the year that Sandberg ceased to be known as Facebook's COO, and became most identified with Lean In, a non-profit and book by the same name that urge women to take a stand in the workplace and pursue their goals.

Lean In may not be a startup, per se, but Sandberg has certainly started a movement, one that will define her career and (I predict) many more new ventures to come.

Max Levchin: From PayPal to Glow

It remains to be seen whether this is one of the best, or simply one of the weirdest, second acts of the year, but it's noteworthy nonetheless. The billionaire co-founder of PayPal announced this year that he was launching a new start-up called Glow, a fertility tracking app that helps women get pregnant. It's certainly an unexpected turn for Levchin, who also sold Slide, the media-sharing startup he founded, to Google in 2010.

So far, the Glow has raised $6 million in funding from Andreessen Horowitz and, not surprisingly, Founders Fund, the venture fund launched by Levchin's PayPal co-founder Peter Thiel.

Kevin Ryan: From Gilt Groupe to Zola

It was just last year that Kevin Ryan stepped down as CEO of Gilt Groupe, the flash sale site he started in 2007, but he didn't waste any time getting to work on a new venture. This year, Ryan, who has also founded AlleyCorp (the parent company to Business Insider), as well as the software company 10Gen, announced he was launching Zola, a startup that's seeking to reinvent wedding registries. Zola allows couples to customize their own homepages and pull in traditional gift ideas from stores, as well as non-traditional ideas, like tickets to an event.

Of course, the launch of Zola means Ryan now has his hand in four different startups at once, enough to make any serial entrepreneur's head spin. As he told Inc.'s Christine Lagorio-Chafkin this October, "It becomes who needs how much when, and how can I provide the most value."

 Jeff Bezos: From Amazon to The Washington Post

He might not be starting from scratch, but Amazon founder Jeff Bezos sure has a lot of work to do as the new owner of The Washington Post, which he acquired in August. In his first post-acquisition interview with (duh) The Washington Post, the big-thinking entrepreneur (did we mention the whole drone delivery thing?) said he plans on advising the team from afar while focusing on his work at Amazon.

"If we figure out a new golden era at The Post... that will be due to the ingenuity and inventiveness and experimentation of the team at The Post," he says. "If we solve that problem, I won't deserve credit for it." So far, he has yet to make any power moves at the paper, however. For more on that, check out next year's list.