In my spare time (what I have of it) I have recently been hooked on Iconfinder. I became an Iconfinder contributor and have been creating and selling my icons on their icon market. Here is my profile in progress!
Iconfinder hosts the worlds largest premium (and free) collection of icons made for designers and developers. You can also commission specific designers to create custom icons for your specific project.
I originally used Adobe Illustrator CC to create the icons and export them as .SVG’s, however I have seen an improved efficiency in my work output by using Sketch. Sketch has very similar functionality to Illustrator however the batch exporting of icons makes the process so much faster. You can create multiple art boards on one stage so that you can export all art boards at once in a file format of your choice, my preference being in SVG format.
If you have some spare time, go and check it out!