In order to build our RIAs with HTML/javaScript I am currently using:

VisualStudio to edit and debug javaScript – it’s still the best editor I have found, but there are some up and comming contenders.

Maqetta as my protyping and design IDE, its a good replacment for Blend. I realy need to drag and drop components on to my UI, as I am not clever enough to do it in code.