Web and application designers need icons, and most of the time we need more or less the same ones---icons representing users, icons for navigation and interactions, social media icons, etc. Several projects have been bundling these icons into pictographic fonts (sort of a Zapf Dingbats for icons). Lately, I've been using a combination of two of these fonts, Entypo and Font Awesome, whenever I need basic, single-color icons.
Both are free and both allow you to make use of the icons for both personal and commercial projects. In addition to bundling together the icons into fonts, Entypo and Font Awesome also provide each set of icons as vector graphics. Having the icons as vectors makes it possible to scale them up and down easily and to create any number of modifications or transformations in an application like Adobe Illustrator. Just the other day, for example, I enlarged and re-colored Entypo's thunderbolt icon for the back section of a book cover.
Symbolset takes the concept of the icon font family even further. Using Symbolset as a font on your website, you can call symbols using common terms that screen readers and search engines see as words. Just type "cart," for example, in your HTML code with Symbolset as the font and a shopping cart icon will appear in the user's browser. This handsome, SEO-friendly icon pack isn't free; the base package is $30. For only $1, though, you can download a set of logos for web-based services and products (the facebook "f", for example) that will surely find a welcome place in your web designer's toolkit.