From the Hugin website:
Goal: an easy to use cross-platform panoramic imaging toolchain based on Panorama Tools.
With Hugin you can assemble a mosaic of photographs into a complete immersive panorama, stitch any series of overlapping pictures and much more.
Since Aperture doesn’t yet include a built-in feature to stitch multiple photos into a panorama, I was looking for a third-party tool today. I found Hugin. It isn’t the most user-friendly software, but it looks powerful and customizable. Most important, it does a great job and it’s free.
Here’s an example from my recent trip to France: