Iconmonstr.com is my personal favorite for download icons for my web applications and desktop projects. The icons are absolutely free for both commercial and personal use but you have to give appropriate credits to the owner.
Like iconmonstr.com, flaticon also contains royalty free images for commercial and personal use but you have to add appropriate credits with the icon.
Iconfinder.com also contains free icons for personal and commercial use but you have to add attribute link to actual author of the icon.
Graphicburger.com is another amazing source that contains free images for personal and commercial use.
These are some of the royalty free sources for icons for your web and desktop applications. Keep visiting this site for more useful information.