Designers always provide awesome PSD's of their work for free, they just want to share their work with others, they want to know they had some sort of impact on other designers. I always make use of free PSD's from Dribbble to see how another designer applied an effect - it's a great way to learn new things in Photoshop.
Here is a hand picked selection of random UI elements. Click on them to go to the source, there you should find download links. This will be a regular series so make sure to come back regularly.