Synchi

Supports both the plugin and the theme editors.

Also, the page/post editor's HTML view is enhanced as well.

Highlighting and editing the code is possible thanks to the CodeMirror tool.

Installation:

Unpack and upload it to the /wp-content/plugins/ directory.
Activate the plugin through the 'Plugins' menu in WordPress.

Features:

• Fullscreen mode support
• Ajax-powered calls
• Line numbers
• Line wrapping
• Highlights matching brackets
• Jump to line shortcut
• Regex search
• Search & replace tool
• Supported languages:
• JavaScript
• CSS
• HTML
• PHP
• XML
• MySQL

What is new in this release:

• Bugs reported by the WP community fixed
• Active theme and active plugins highlighted in IDE file explorer
• Line wrapping now optional through settings
• Security improvements

What is new in version 5.0:

• Added syntax highlight for text widgets in widget editor.

What is new in version 4.3:

• Added extra settings for indenting with tabs and controlling tab size
• Updated CodeMirror source to version 2.32
• Fixed focusing on first tab when initializing IDE
• Fixed 'Save All' bug when no changes are made

What is new in version 4.2:

• Added Ctrl+Space code completion for php and javascript files (including WP php functions)
• Added matching words highlight
• Added uninstaller to remove junk when removing plugin
• Secured Synchi request handler to ensure that no one but admin can use Synchi

What is new in version 4.1:

• Added options to enable/disable Synchi in plugins & themes editor and article editor.

What is new in version 3.3:

• Added Ajax document saving for themes and plugins editor.

Requirements:

• WordPress 2.7 or higher