Axure, the widely used wireframing tool, issues free licenses to students and teachers:
If you are a student enrolled in a high school, university, community college, or extension/certificate program, you are qualified to receive a free educational license of Axure RP Pro.
If you are a teacher interested in using Axure RP Pro for a class, we provide an educational license for you and a second educational license for use on classroom or lab computers.
Axure RP Pro is a wireframing, rapid prototyping, documentation and specification software tool aimed at web and desktop applications. It offers drag and drop placement, resizing, and formatting of widgets. Axure supports prototyping Rich Internet Applications by mapping desired interface behaviors (such as displaying or hiding an element) in response to actions like mouse clicks or touch gestures. Axure RP generates HTML web sites and Microsoft Word documents as output.