I find it interesting that every piece of software suggested in this topic is solving a different problem. When you say "Mosaic" to me, that brings to mind gluing together dozens or even hundreds of individual images in a pleasing manner with no gaps. The images could be overlapping or tiled. Basic photo editing software such as GIMP or Photoshop can handle this, but the overhead of trimming each individual photo may be more labor than you want. For each image, you would import it as a new layer. Eventually (as you go or every so often or even just at the end) you have to "flatten" the multiple layers down into one.
There are at least two other concepts discussed in this topic. One is to stitch together multiple photos (of one scene) into one large panorama, and one is to treat photos as mosaic tiles, where each photo is shrunk to a small size and forms almost a "pixel" in the finished picture (usually replicating another photo). Both could be more or less automated. There is also photocompositing, as used in astrophotography, to rotate, shift, and align up to hundreds of exposures (including webcam frames) to build up a high quality image. There are applications to assist in that, but usually some manual intervention is needed.
So, in the end, what kind of "mosaicking" did the OP want? If you're going to suggest a solution, be careful to state what this solution does.