WP-Syntax provides clean syntax highlighting for embedding source code within pages or posts.
Try editing code snippets without the visual editor. To turn off the visual editor for all your edits, uncheck the visual editor checkbox in your profile. Depending on what type of code you're trying to display, you might also need to disable WordPress' corrections of invalidly nested XMTML (under Options -> Writing).
By default, WordPress filters HTML for particular user roles, and this affects WP-Syntax's input. As a workaround, install the Role Manager plugin, and check "unfiltered HTML" for the roles that would like to post code snippets.