Icons for applications is like Salad dressing without it either desktop or web applications is tasteless and as it is always said “A picture is worth a thousand words” and if you are a website owner or designer then you should use icons where appropriate to deliver ideas to your visitors or catch their attention.
In this article I will list a few links where you can visit to search and download professional free icons and if by the way you are an icon designer as well then I will direct you to submit your icons on some websites that accepts submissions. You can always buy professional icons from websites like graphicriver.