Enterprise may not be on your radar because the company is privately held. But it’s remarkable in many ways: Enterprise is the world’s largest rental car company, with revenues approaching $13 billion. (You thought Hertz and Avis were No. 1 and No. 2? Enterprise was bigger even before it bought National and Alamo last year, and now it leaves those more famous names in the dust.)

Not only is Enterprise growing. It hires more entry-level employees than any other in the U.S. Just behind it on CollegeGrad.com’s 2008 survey of the biggest recruiters, released in June: AmeriCorps, Walgreen , the IRS, and Progressive , the insurance company. Target , Lockheed Martin and Bank of America rank among the top 20. But Enterprise, which puts new hires to work scrubbing cars, recruits thousands more entry-level employees each year than any Fortune 500 giant.

Nicholson, 48, launched her career like most any successful Enterprise exec: behind the sales counter, as a management trainee. She started in the company’s hometown of St. Louis after graduating from the University of Missouri with a BA degree. Now she’s considered a contender for the CEO position.

My colleague Carol Loomis’ 2006 story about Enterprise details the the company’s unusual low-cost business model – operating mainly outside of airports – and down-to-earth Midwestern culture. This culture is all about hard work, intense customer focus, and an innovative incentive pay system. Branch managers are compensated based on the profit at their individual operation. If you’re a great entrepreneur, you thrive.

At a time when turmoil and ugly politics have swirled around so many corporate suites – and helped topple powerful women such as Zoe Cruz at Morgan Stanley , Erin Callan at Lehman Brothers and Pat Russo at Alcatel-Lucent – it’s good to see a woman climb the ladder safely, the old-fashioned way.