many eyes clone: Write an open clone of the many eyes visualizers.

google finance clone Write an open clone of the Google Finance flash widget. Possible starting point: http://www.measuremap.com/developer/slider/.

Generic Six Degrees: Write a generic Oracle of Bacon so that you can drop in any large graph (like this graph of book relations) and get back "how do I connect A to B", "what's at the center of the graph?" and other nice features.

Decent chart library: Anyone got a decent charting library?

Code samples: Graphviz, Matplotlib, Matlab, Numpy, whatever you've got.