CodeColorer is the plugin which allows you to insert code snippets into the post with nice syntax highlighting.

Plugin based on GeSHi library, which supports most languages. CodeColorer has various nice features:

line numbers
automatic links to the documentation inserting
code block intelligent scroll detection (short code would have short block, for long one block height would be fixed and scrollbar would appear)
predefined color themes (Slush & Poppies, Blackboard, Dawn, Mac Classic, Twitlight, Vibrant Ink)
syntax colors customization in CSS file
syntax highlighting of the code in comments
code protect from mangling by WordPress (for example, quotes, double-dashes, etc would look just right as you entered)

http://wordpress.org/extend/plugins/codecolorer/