Pity the poor sports world: It has only one Green Monster, and while Boston is a great place, it's not convenient for everybody.

On the other hand, the investment world has plenty of green monsters -- companies that pile up the cash on the balance sheet. Some companies get the bulk of their cash by selling assets or shares, but the great ones do it with cash from operations. These companies pack a double punch, not only because they have the cash to pay dividends, buy back shares, or invest for growth, but because they've got more cash coming in the front door to replace what is spent.

Microsoft (NASDAQ:MSFT), Berkshire Hathaway (NYSE:BRK.A) (NYSE:BRK.B), and Apple (NASDAQ:AAPL) are all famous green monsters. FactSet Research Systems (NYSE:FDS) is less famous, but also rakes in the cash. But, my favorite green monster is the Hidden Gems purveyor of products that help you breathe easier, CNS (NASDAQ:CNXS).

Few companies back up one-third of their market cap with the green stuff and still sport a trailing price-to-earnings ratio under 20 and a forward P/E under 15, but CNS does. Considering the solid growth the company accomplished last year, it ranks as a value in a market where value is tough to come by. Lately, CNS is also getting more efficient at spending the cash it generates. Proof is in the 1% increase in sales and promotion expenses that netted 10% net sales growth and 31% net income growth.

CNS is still not perfect, though, as it depends on Wal-Mart (NYSE:WMT) for 26% of its sales. This is a bit higher than I'd like, given the retailing giant's reputation for bullying small suppliers. But I like CNS's odds in this battle, because this green monster dominates the better breathing category, and customers who have come to rely on CNS products would most likely go to competing retailers to pick them up.

Few things in life are certain, and even a green monster can run into trouble, but when it does it can also weather the storm. At CNS's present valuation, even if sales were to flatten out a bit -- which is not expected -- the downside should be minimal from here and there is still plenty of room to grow. In time, this hidden gem has a good chance to really sparkle.

Fool contributor Nathan Parmelee is a big fan of the 37-foot tall Green Monster and the one that lives in his portfolio: CNS. He also owns shares in Berkshire Hathaway, but none of the other companies mentioned.