Inline Preview for WordPress
I’ve written a plugin that moves the post preview process into the editing screen instead of in a separate tab: Inline Preview.
With the plugin installed, if you click Preview on a post or page, a preview iframe will slide in from the right side of the page instead of opening in a new tab. There’s an X to close the preview pane, and you can resize it by dragging on the left edge of the preview. Clicking Preview again will reload the iframe.
I’m not convinced that this kind of preview is the right way to go in the long term, but after using it for two days, I like it more than the current preview implementation.