WP-Syntax provides clean syntax highlighting using GeSHi — supporting a wide range of popular languages. It supports highlighting with or without line numbers and maintains formatting while copying snippets of code from the browser.
It avoids conflicts with other 3rd party plugins by running an early pre-filter and a late post-filter that substitutes and pulls the code snippets out first and then pushes them back in with highlighting at the end. The result is source code formatted and highlighted the way you intended.
Usage, Supported Languages, Styling Guidelines, and Release Notes are availabe in the Other Notes section.