Founded in 1969 and based in Little Rock, Ark., Acxiom runs a vast collection of businesses ranging from background screening to social media marketing. Only rival ChoicePoint does anything close to the scale of Acxiom's business.
What they know: Pretty much everything. The company tracks all kinds of personal information, from your Web behavior to your Social Security number and your finances. It buys information from many other data aggregators, privacy experts say, so it has all of the details every other data miner has about you -- and it has more information individually than any other company.
"Acxiom is Rome," said Pam Dixon, executive director of World Privacy Forum. "All roads lead to Rome."
How to opt out: It's more complicated than you might think. To opt out of online advertising, click here.
To stop getting direct mailings and to take yourself off of Acxiom's other directories, you need to fill out an online form. The company will then snail mail you another form that you need to send back for "processing."