There's a new demo of Jonathan Edwards' Subtext system online. This is the best example I've seen yet of an extensible programming system. It's pretty cool---once you stop insisting that programs have to be directly presentable as ASCII strings, many new thoughts become possible.
Quote: "It's good to remember that with practice, you can even learn to read Perl. Humans are not good at abstract thinking, but they're great at pattern recognition. I think it's a win to trade one for the other."
(And to follow up on the "many new thoughts become possible" meme, I once saw a talk by Richard Dawkins on "The Evolution of Evolvability". Dawkins' thesis was that some evolutionary changes give evolution new things to act on---segmented bodies, for example, allow for specialization of segments and limbs. Code-in-database systems almost certainly have this property.)