Do you have a Canon camera? If so there's a applicaiton on the disk that came with it named PhotoStitch. It works very well. I know Photoshop can stitch and maybe Photoshop Elements for Mac (which would be the the one to choose if it did). I've seen others mentioned but don't remember what they were.
You might try going to MacUpdate.com and search for "panorama" or "panaoramic" to see what comes up.
Some cameras come with Panarama Maker, another great app for Photo stitching.
Or try the free good, free panorama-maker "Hugin".
It is a very good open source program, running on many platforms; but the documentation is not very intuitive