A software/design/consulting firm in New York City, "Plumb Design creates online experiences that facilitate the exchange of knowledge and the interplay of ideas."
Some of their clients include Merrill Lynch, Experience Music Project, the Smithsonian Institution, and Motorola. One of the most commonly cited examples of their work is the Plumb Design Visual Thesaurus.
Plumb's subsidiary Thinkmap develops kick ass software for visualization of complex sets of interrelated information.