I've been saying for a while now that extensible programming systems are the Next Big Thing, but I wasn't expecting them to arrive this quickly. Check out Expressive Programs Through Presentation Extension, by Eisenberg and Kiczales (at UBC). They use Java plus annotations for storage, rather than XML, which allows them to piggyback on the standard Java/Ant/JUnit/Eclipse toolset. They even provide (simple) graphical embeddings; the only thing missing from the core I asked for in 2004-05 is debugger support. Anyone wanna do a PhD? ;-)