Bill Gates evidently made a New Year’s resolution last year — one that he didn’t say much about.

Of course, when you’re Bill Gates you don’t have to say much to make the things you resolve happen.

In fact, Gates may have only said one word: “Buy.”

And “buy” is exactly what he did. In fact, Gates made the biggest insider purchase during the past year by an individual, more than $400 million worth of one company.

Hint: It’s not Microsoft (Nasdaq: MSFT). Gates is offloading shares of the company he founded in million-share blocks. He’s been selling his stock for years and will keep doing it, not because Microsoft is a bad company to own but because concentrating one’s wealth into a single security, even a blue chip like Microsoft, is never a good idea.

The company Gates started buying in January, and continued buying in 500,000-share lots through early March, was Republic Services Inc. (NYSE: RSG). Gates had to report the holdings because he was already a 10% owner — an “insider” in Wall Street parlance — of the company. Gates started the year with 36.5 million shares. He now owns 55.4 million.

Republic is a trash company, the No. 2 player in the $52 billion-a-year domestic non-hazardous solid waste industry.

The company provides garbage collection services for commercial, industrial, municipal and residential customers through 400 companies in 40 states. It also owns 213 solid waste landfills — that is, “dumps,” — and 78 recycling facilities. Republic is a national company that basically operates on a local level.

Public companies control about 58% of the trash collection market in the United States, a segment worth some $30.1 billion. Republic’s $8.2 billion in 2009 revenue means the company has about 28.2% of the available market. Outside of Waste Management, however, the market is highly fragmented, which means that even though garbage collection is a mature industry, there is still room for growth.

Gates not only bought these shares for his own account, he’s also stuffed the coffers of the Bill & Melinda Gates Foundation with them, about $35 million worth. The foundation owns $468 million worth of Waste Management Inc. (NYSE: WM), the foundation’s largest single holding outside of Berkshire Hathaway (NYSE: BRK-A), which Warren Buffett is donating.

The question, of course, is why did Gates buy? And, further, is this stock currently a worthwhile opportunity for investors?

Answers: Gates bought because Republic is cheap. And you should, too.

Gates’ best price for Republic shares was $18.80 a share for a 731,700 share block on March 2, as the market was circling the bowl. He’s earned +53.8% so far on those shares and even seen a +9.3% increase on the shares he paid the most for, a $10.8 million acquisition of about 410,000 shares at $26.45 in late January

Though he’s done well so far, the best is yet to come. Republic’s 2010 earnings should come in at about $1.68, which, at the company’s typical valuation, gives them a fair market price of $43.18, or +49.3% ahead of where there are today.

This outlook was evidently enough to convince at least one other major investor to buy in — some guy from Omaha that Gates plays bridge with.

Buffett’s Berksheve Hathaway now owns 3.6 million shares, according to its most recent filings with the Securities and Exchange Commission.

My prediction for 2010 is that Gates and Buffett — old friends: Gates sits on Berkshire’s board and the two have played bridge for years — will continue to buy into the trash business. Gates is already a huge player, and Buffett can be any time he wants.

It makes some sense, given the Oracle of Omaha’s penchant for simple businesses with few additional capital requirements that can be bought for a reasonable price and deliver results for decades to come. Trash collection promises a stream of steady, noncyclical revenue, and though Republic looks pricey at 44 times trailing twelve-month earnings, it looks Buffett-style cheap looking forward to anticipated 2010 results.

P.S. Insider buying is just one of several “profit catalysts” that can send a stock to the moon in a matter of weeks. Others include a surprise takeover announcement, a killer new product, improving business conditions, geopolitical shifts, and more. Using this “profit catalyst” approach, my colleagues Nathan Slaughter and Paul Tracy are picking winners with nearly 90% accuracy — and generating gains of up to +565% for their readers. Don’t miss their next recommendation — it’s being released in a few days.