Pinterest

Pinterest is a social media web and mobile application company that operates a software system designed to discover information on the World Wide Web, mainly using images and, on a smaller scale, GIFs and videos. The site was founded by Ben Silbermann, Paul Sciarra and Evan Sharp.

Freepik

Freepik is a great source for illustrations and graphic design. I have personally used to it to create very beautiful and detailed infographics. What's great about Freepik is that the content can be used in both personal and commercial projects (with a link to Freepik).

Dribbble

Another great source for inspiration, Dribbble is a community of designers that share with each other, other fellow designers, what they are working on. The community consists of web designers, graphic designers, illustrators, logo designers, icon artists, typographers, and other creatives which share screenshots of whatever project(s) they're working on, making it a great destination for attaining new ideas for your projects.

Iconfinder

Based in Copenhagen, Denmark, Iconfinder works with a community of local and international icon designers. They are continuously expanding and adding to their very large collection of icons available for download. Some of their icons are free to use without permission, others are free to use with attribution to the author, and others are for sale, but for very reasonable prices (usually $1 or $2).

Google Fonts

There are many font websites on the Web, but most of them you need to pay for the rights to use the fonts for commercial use. The wonderful thing about Google Fonts is that their entire catalog of fonts is free and open source, accessible to anyone for any project, both personal and commercial.