As a part of the Adobe Creative Cloud suite of apps, XD is built specifically for creating UI and interactions. Free to use if you're a University of Arizona student.

Powerful vector design tool that lets you collaborate right within your browser. Free education plan for students and educators. 

Long-time favorite by many designers thanks to its huge arsenal of plugins. $99 to use ($49 if you're a student), and it only runs on macOS.

Handy design tool for creating social media graphics and posters. As part of the GitHub Student Developer Pack, Canva offers free pro plans to students.

The world's best community for high resolution free-to-use images.