This weekend I tried out the Eclipse Graphical Editing Framework. First impressions are that it is a very rich, mature, extensible framework. Like Eclipse itself, there is enough decoupling and extensibility hooks that it takes a while longer to get a basic application up and running; but once you've reached that stage you can keep adding features in an extremely flexible way.
With its model, parts, figures, policies and commands, GEF makes MVC look tightly coupled. The two examples that come with GEF are excellent examples, although I found they were a little too advanced to base my first application on. At some stage, I'll post my first application with documentation as I think it might provide a useful stepping stone to a more complex example.
Look out for some future open-source projects from me based on GEF.
UPDATE (2004-11-03): Now see Six Snapshots of a Simple Eclipse GEF Application.