Forbes 400
Top 10 this year?
5 hi-tech (Gates - MSFT, Brin/Page - Google, Ellison, Dell)
2 oil (Kochs)
2 casinos (Adelson/Kerkorian)
1 Buffett!
It's probably a little disingenuous to classify KK as casinos, since that's really just part of his investment/buyout portfolio, but it's a big part of his roots, focus, and empire.
Rounding out the top 20 are a couple more Microsoft guys, the Waltons (Wal-Mart), and a couple of finance players (Icahn, Fidelity). And the Mars bar family!