iconfu is a free online tool that lets you create professional looking icons instantly. You can select to draw from various available tools Normal Pen, Alpha Pen, Color Sampler, and much more. It is totally customizable, you can change the size of the icons, add transparent effect, create shadows, choose from thousands of colors, etc.
Every change is shown in the small preview window, making any change to the icon creates a new version of it, you can select these versions from the left sidebar. One great feature is that you can add animated effects to the icon. You can save the icons in three formats, png(which is great if you want to keep the icon online), gif(which can be used to save animated icons), and ico(which is Windows default format for icons).
Registration is not required at all, but if you registered users get additional features and benefits to save this icons online. Enjoy!