A visual thesaurus describes the relationships between words as a three-dimensional network, letting you interact with their relationships, traverse them, etc.
AllConsuming does something similar for books, and, tangentially, for ideas. It tracks book links on blogs, so that you know that persons A and C are reading the same book, and can infer that they’re thinking similar thoughts, or working on similar projects, and so forth.
I wonder if you could find a thesaurus for concepts, or maybe an encyclopedia of ideas that pointed out the relationships between them: derivation, opposition, refinement, sub-categories.
Secretly Ironic 