PIE stands for Progressive Internet Explorer. It is an IE attached behavior which, when applied to an element, allows IE to recognize and display a number of CSS3 properties.
Basically it is a “fix” for IE browsers that don’t support some of the fancy new CSS3 elements.
As this was up on my browser, one of my coworkers who had tried it, immediately said “sucks”. I asked him to elaborate and he said it is half-way decent, but is not fool-proof. It doesn’t function perfectly and some things are basically ignored.
I would still use Modernizr, but I will at least give this a test try.