After I told a colleague about the Calkin-Wilf tree (see Enumerating the Rationals in Python), he suggested a program that would generate the tree graphically.
I decided it was a good excuse to try writing my first Eclipse 3.0 rich-client platform (RCP) application. Given the children of each node in the Calkin-Wilf tree are derivable solely from the node itself, it just took a little tree content provider and the RCP skeleton pretty much copied straight from the example app referenced in the online doc.
It worked out pretty well. Writing the application itself was straightforward; packaging it up with the necessary components from the RCP platform was not. It took a lot of trial and error to get the minimal set of additional plugins and to bootstrap the execution of the application outside of the IDE.
What I would like to see (and I'm sure will be in 3.1) are some improvements to the plugin development environment specifically for RCP; namely, a wizard for creating the base skeleton (plugin, app class and advisor class) and a wizard for packaging the RCP app for standalone execution.