You have to look at real rates, not nominal rates. The only markets that have negative nominal rates are battling deflation (which the US is not) or have serious liquidity concerns at the moment. The Fed is unlikely to go negative as they face a very different beast.

Here's a quick example. Let's say you're in a deflationary environment: in 1 year, your money actually buys you more than it did last year, let's say for instance, 2% more. Under this weird environment, you would actually be willing to pay someone to hold your money for a year, because you know in a years time, it will buy you 2% more (effectively a 2% return). So, since you don't want to store that money under your mattress, how much will you pay someone? Let's say you pay your bank 0.5% for holding your money (i.e. negative rate). So in a years time, you get 99.5% of your money back from the bank, but it buys you 2% more "stuff" so really, compared to today, you're getting 101.5% (roughly) of your money back, which is the same as a 1.5% interest rate. This is called the "real" interest rate, and it's the only one that matters.