Microsoft co-founder Bill Gates is once again the richest person in America. Gates sits atop Forbes' list of the 400 wealthiest Americans with a total net worth of $81.2 billion - roughly $13.4 billion more than the next richest American on the list, Warren Buffett.

The fact that Gates has retained such wealth is telling when you consider he's donated nearly a third of his money - $30 billion - to charity through the Bill & Melinda Gates Foundation. That much money on its own would be the 16th most in the US.

Interestingly enough, the Microsoft co-founder added $9 billion to his net worth in the last year alone. As Forbes points out, the gain was partly due to a sharp rise in shares at Microsoft. Adding to that is the fact that Gates has enjoyed solid returns on his investments in Canadian National Railway and trash company Republic Services.

It may also come as a surprise to some that Gates owns a significant amount of stock in tractor maker Deere & Co. and South American McDonald's franchise Arcos Dorados.

If nothing else, Gates' fortune should serve as an example of how to invest - don't keep all of your eggs in one basket. By that, I mean diversity is key to a successful portfolio. If Gates had his entire fortune tied up in Microsoft stock, he'd be doing really well right now. But on the flip side, Microsoft (just like any other company) could tank at any time and if that happened, he'd be looking at a significant loss.

Rounding out the top five is businessman and investor Warren Buffet with a net worth of $67.8 billion, Oracle co-founder Larry Ellison at $48.1 billion and businessmen Charles and David Koch at $42.9 billion each.