Since this tool would be available to numerous students, it would be very helpful that a wide array of related topics are mapped out so that each student focuses on what's most relevant to their interests/needs.
Prezi is a good resource for students to look at a wide array of related topics and choose the angle that best suits their research or class direction. Gradeguru and Notehall are mainly about student exchange of helpful notes and study materials. Dropbox is mainly a storage tool that also allows sharing and exching files. Chegg is a book rental tool.