Microsoft made the PhotoSynth application for iPhone but forgot to create one for OS X.
How do I stitch several photos into single one, free apps would be preferable.
I found Hugin to be the best free alternative to PhotoSynth. I only used the Windows version but they have Mac builds too.
PTgui is a commercial alternative to Hugin using the same core library. I remember giving it a spin and not being impressed over Hugin.
Another commercial alternative is Autopano (started as a free GUI to panorama tools and went beyond that with PRO and Giga versions). I has a more polished look and feel and the Giga version is targeted at really large panoramas (think 26 gigapx).
I've become acquainted with a few models of Canon and Nikon point-and-shoot digital camera; all models in both brands came with panorama software. If your digital camera came with a software CD, you might stick it in and see what it installs.
DoubleTake is not free but you can use the demoversion. I use it to stitch together not photos but screenshots and similar graphics. Most panorama creatings programs does not work very well with "GIF-like" graphics and typically expecting horizontal rather than vertical stitches but DoubleTake does an excellent job with that.