PluGeSHin is a WordPress plugin by Peter Ajtai that lets you use the syntax highlighting of GeSHi through WordPress shortcodes.
A typical view of the WP-Admin screen when using PuGeSHin. You can use either the View or HTML modes, but I find using the View mode easier.
The highlighting shown on the front end due to the shortcode tags used in Screenshot-1.
The PluGeSHin TinyMCE split button at work
The PluGeSHin modal
The grammar for the PluGeSHin shortcode. Thanks to the Railroad Diagram Generator for the diagram!