Brackets, i.e. I now love HTML/CSS
They say they're working on JS changes popping through immediately in preview mode. I also see LESS and SASS add ons that claim to compile either immediately or on save. Would love to have reliable support for both JS and one of the preprocessors.
Other nice features include decent autocomplete, theme/highlighting support, file navigation, and similar fairly-standard stuff. Plus, there's inline styles discovery and editing. I.e. you can put your cursor at an id and see all of the styles that relate to it and edit them in a little accordion that pops out in your HTML to reveal the right part of your CSS.
All in all, a pretty sweet tool that made my day.