Thank you, University of Adelaide. It turns out that a researcher there, Professor Kym Anderson, has been engaged in a lengthy project analyzing the world's grape-growing regions and has determined—among many, many other things—that Cabernet Sauvignon is the most widely planted wine grape in the world.
Of course, there are variations by region—in Kazakhstan, for instance, Rkatsiteli is the most widely planted variety. In Thailand, somewhat mysteriously, it's Syrah. But overall, Cabernet wins. Be glad. Twenty years ago the world's most planted grape variety was a Spanish white grape called Airén, notable primarily for being incredibly bland.
And so, seeing as how there's so much darn Cabernet in the world, a little advice about which ones to buy seems in order. Here are a few top bargains. (And, if you truly want to indulge your inner wine geek, Anderson's wine-grape study is available for free as a PDF e-book.)