Inline Preview adds a preview of the post you're writing next to the editor when you click Preview (instead of opening it in a new tab).
Inline Preview is a plugin for WordPress that adds a live preview of the current post next to the post editor instead of opening the preview in a new tab.
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.
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.