Problem: When creating a post in WordPress, previewing your post in a separate tab can be slow and/or complicated.
Solution: Show the preview right next to the editor.
Inline Preview is a plugin for WordPress that adds a zoomed out preview of the current post next to the post editor when the user clicks Preview instead of opening in a new tab.
It's just an idea I'm playing with for streamlining the post creation process, and if it feels familiar, it's probably because it is: through version 2.1, WordPress contained a preview iframe at the bottom of the post editing screen, but this is a more elegant implementation. Feel free to install the plugin and try it out for yourself.
Update: I've updated the plugin to add a close button (a translucent "X" in the upper-right of the preview) and to make the preview frame resizable by dragging its left edge. The new width of the preview is persistent each time it opens, even across pageloads.