elsewhere on HN today is a short article on BBedit, which has a really nice shell worksheet interface similar to the old MPW. Select the text you want to execute, hit control-return, and the shell output will appear below the selected text.
If I'm not mistaken, I believe this feature originates with Smalltalk (you might think that Smalltalk is a language and the feature in question is an editor feature, but most Smalltalk variants are tightly integrated with an editing environment).