Of course, Radiohead, famously. And recently World of Goo did very well by it and publicized all their numbers (very cool). Real-world restaurants seem to be running with it, and bars as well! And cab drivers, even. :) Lotus (the band) toured that way. Though Pay what you want online is a music service that charges a minimum of fifty cents a track--that's not very "what you want". A hotel. Here's a one-off font that starts at $15/yr min (I think that misses the point?) And Eidos is offering Championship Manager 2010 for just a penny (plus 2 pound fifty transaction fee--haven't they heard of micropayments???).
Digging deeper, Paste Magazine beat us to the "magazine" punch. Faber did it for an ebook (or more?).
GUD Magazine is trying it as a sale...
It's a crowded market of want, but really, how wrong can you go paying as little as a penny?
The question's not entirely rhetorical, as we haven't had hordes knocking our doors down to get their penny PDFs. Exposure, sure... and we've not sold "nothing" from it in the day or two it's run so far--people almost seem embarrassed to not pay a "fair" amount, but we'd rather they paid less than that and got a copy (or a copy of all five, for that matter).
Update: not hordes knocking down the door, but it's still our best sale by a wide margin. I'm very hopeful it will continue to improve. If you can spread the news, I'd really, really, really appreciated it. :) If you haven't already. ;)