I love using the free icon set Font Awesome in my web applications, and I know I am not alone. A new major version is in the works, and the creators are using Kickstarter to help fund it. Font Awesome version 5 will still be free, but as an early backer you get the Pro versions, with over a thousand extra icons, and much more. And the price? Only $20 until the end of the Kickstarter tomorrow Monday at Noon Eastern time. After this the Pro version will be $40 and each of the currently 18 icon packs (each with 30 icons) will be $10. So this is a substantial savings.
If you are doing any kind of web development and not already using it, you should take a closer look at Font Awesome.