two years ago when i was living in hawaii hubby and i did a last minute cruise through a deal on cruisedirect.com. we booked it on a thursday or friday and left the following monday, $365 (or $375) for a 10 night hawaiian cruise. ^ super deal.
of course, we were in an inside cabin next to the ship's dancers, who argued long and loudly with their boyfriends at 2am, but we had a fine time and it was a fun holiday nevertheless.
as other posters have pointed out, it's not a frequent occurrence, but if you live driving distance to a port, (or have funds for last minute airfare) there are a few deals out there.