You know about Gates. You know about Buffett. But do you know about Slim?

Mexican telecom titan Carlos Slim Helu quietly slipped past value investor Warren Buffett in late March to become the second-richest man in the world. Slim is worth $53.1 billion, compared with Buffett's $52.4 billion, according to our rankings. He is also breathtakingly close to passing Bill Gates, currently worth $56 billion. Gates, who co-founded Microsoft in 1975, has been the world's richest man for a record 13 years.

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Slim added $4 billion to his fortune through stock market gains in the two months before we locked in net worths for our annual billionaires rankings. It's an amazing run for the 67-year-old tycoon. He added $23 billion to his fortune in just over a year. The surge was fueled largely by a strong Mexican economy and a stock market that jumped 49 per cent in 2006.

Yet surprisingly, many outside of Wall Street or business don't know him. Like many other fantastically rich people, he has slipped under the general public's radar. Unlike Slim, that's often because they built their wealth through privately held companies. Take the extra-secretive Cargill family. The descendants of William W Cargill, who started an agricultural empire with one grain elevator in post-Civil War Iowa, keep a low profile.

That's because they can. The largest private company in America, Cargill is still 90 per cent family-owned, and employees own the rest. Unlike Gates, who must disclose his Microsoft stock holdings in public filings, the Cargills are under no such obligation.

There are two branches to the Cargill family. After William W Cargill's death, his son-in-law John MacMillan rescued the company from a debt crisis. Today, Forbes estimates that the MacMillans are worth $1.2 billion each. Last year, the fortunes of James and Margaret Cargill were estimated at $1.8 million each, but both passed away in 2006, as did W. Duncan MacMillan.

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Of course, most billionaires are well known within some communities, so we can't promise that you've never heard of the rich people on this list. Wall Streeters undoubtedly know about Abigail Johnson, whose family controls Fidelity Investments, the largest mutual fund company in the US. Johnson graduated from college in 1984 and started working at the family company four years later. She ran her first diversified fund in 1993 and became president of the company's mutual fund division in 2001. Net worth: $12.5 billion.

And Albert von Thurn und Taxis, the youngest billionaire in the world, is well known in his home country of Germany. He inherited a fortune on his 18th birthday in 2001. His assets now include an art collection, a tech company and one of the largest parcels of forest land in Europe. He lives in one of his family's castles. And yes, ladies, as far as we know, he's single.

Then there are the emerging-market billionaires, who are well known in their home countries and will soon be making headlines all over the world. Take Vijay Mallya, who may be the Indians' answer to Richard Branson. Right now, the flamboyant liquor mogul is worth only about $1 billion. But Mallya is growing his fortune by expanding his liquor business and, more recently, by starting an airline.

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His UB Group acquired archrival Shaw Wallace [Get Quote], becoming the third-largest spirits producer by volume. Top brands include Bagpiper whiskey and Kingfisher beer, and he launched Kingfisher Airlines in 2005. He has a fleet of vintage cars, a stud farm with 200 horses and an 18-year-old son, Sidharta, who has been anointed his successor.

As for Slim, no doubt he's headed toward becoming a household name. The Mexican magnate's rising fortune has caused a good deal of controversy because it has been amassed in a nation where per-capita income is less than $6,800 a year and half the population lives in poverty.

Critics claim he is a monopolist, pointing to Telmex's control of 90 per cent of the Mexican landline telephone market. Slim's wealth is the equivalent of roughly 7 per cent of Mexico's annual economic output. If Gates had a similar proportion in the US, he'd be worth $874 billion.

Slim says he is unfazed by the criticism. "When you live for others' opinions, you are dead. I don't want to live thinking about how I'll be remembered," he said earlier this year. He also claims indifference about his ranking and says he has no interest in becoming the world's richest person.

When asked to explain his sudden increase in wealth at a press conference soon after Forbes' annual billionaire rankings were published, he reportedly said, "The stock market goes up ... and down," and noted that his fortune could quickly drop.