You need or want to build a mock up with photoshop to present your app in a device, a webpage on a mac book or something like that?
Today I hit pixeden and I am really impressed. They offer commercial and free PSD templates in a very good quality. So whenever you need to build a mock up, take a look around. At least for mac devices you should find something useful:
If you look specific for free templates, Pixeden offers a specific category for that (not mock ups only):
Photoshop Mock-Up Templates