MediaLoot made a free to download collection of vector clouds for Photoshop. Included are 18 individual vector clouds and 1 scalable vector skyline background.
Awesome collection of free psd files provided by djdesignerlab.com.
40+ Useful Free PSD Files
Some days ago on splashnology.com I found this useful article that lists few free UI Elements from different websites.
I found this interesting list of free PSD button designs. Everytime that you need a button for a website can you try looking in these sites: