Update: I actually don't even have the car anymore. But I did immediately change this tire at the time.
It's my rear right tire. It has a significant bulge in the sidewall.
Here are two pictures of the bulge (original URL):
Do I need to replace it before I ever drive on it again? Should I put on the spare to go buy the new tire? Or is it fine to even go a little bit on the highway for a few days before I replace it?