Hi and welcome aboard
Needless to say that Photoshop is the best software you can use to achieve what you want. You won't find anything that can compete, whatever people could say. It's the best. Period.
On the other hand, it's not free but Gimp is, and available on Mac osx (http://www.gimp.org/macintosh/ )
It's not as good as PS but it is free which is quite nice to say.
Depending of the outcome style you want to get, Gimp can be enough but even if there some plug-ins that can add more flavor to the program (which can add the layer styles you have in Ps, canon) you won't get the same versatility. Give it a try, a least
I've never used any other program so I can't advise you more