I'm looking for a relatively easy way to take several portrait photos of faces and then blend them together into a coherent face. For example, I'd like to create androgynous faces by combining pictures of both men and women.
there are lot of good freeware and licensed soft. I recently use Sqirlz Morph (freeware on Windows) that allow to morph several source together, as you specify, with relative good and fast result (for my point of view and constraint). There is several output format allowed from still image to video sequence (without sound :-D).
The best bet is to choose some software that has a demo and try it out. There is a wide variety of software available and the best choices depend a lot on what you want to accomplish. The more advanced software is harder to use but will do a better job overall. Other software does it very easily, but doesn't do as good of a job. Personally, I used to use a package I think was called Morphman that worked pretty well and was fairly detailed.