Paperscape.org presents a really interesting dynamic visualization of the connectivity of papers within the arXiv. Each paper is a small disk, whose radius is proportional to number of times it has been cited, colored by subject area. The x, y position of the set of dots constructed by some kind of algorithm that clusters them by how they reference each other. Overall it is an effectively useful map to a significant portion of scientific knowledge. I definitely want to use it the next time I start a literature search on a topic.