It is a great resource for free icons and png image archive. I found everything I was looking for my projects. Really very useful.
Pixabay - Over 270,000 free photos, vectors and art illustrations
Freepik - More than a million free vectors, PSD, photos and free icons. Exclusive freebies and all graphic resources that you need for your projects
Flaticon - A database of free vector icons.
FreeImages - Find and download free stock photos - all free for personal and commercial use
PNGTank - Gallery of thousands of free PNG images with transparent background
Pexels - Find the best free stock images about Browser Home Page. Download all photos and use them even for commercial projects.