Zero

Treasury bill interest rate dropped to or below zero. This is on the four-week treasury bill.

Essentially what this means is that investors are "lending" their money to the government and not expecting any kind of return on it. They give $1,000 and four weeks later they are given $1,000. Or maybe $999 if you believe the Wall Street Journal. Essentially the four week t-bill system is a giant mattress where occasionally a dollar bill falls out.

EDIT: Whoa, apparently YouTube is no longer blocked, so I can listen to whatever the hell I want to. Sweet.