Freebbble!

Fully scalable Photoshop vector shapes, in two angles (from left and right).

Freebie! 27″ iMac PSD is a free Mockup design resource made by Dribbble user Breezi. As far as I can tell, its license is Creative Commons Attribution.

Helpful hint: Dribbble users are nice. If the license is unknown, it is usually because they forgot to mention it explicitly; not because they were being stingy. Ask nicely and they will probably allow you to use it.