WP Utility Short Codes is a WordPress plugin that helps you easily add callout boxes to your posts with simple shortcodes. (Some examples here.)
Using WordPress short codes, this plugin will allow you to easily insert message boxes and buttons into your WordPress posts and pages. With easy to use options, there are over 1,000 possible display configurations. All buttons and message boxes include multiple color and size options.
I haven’t tried it out myself, but it seems like a good plugin. The code is very straightforward, too. It shouldn’t be too difficult to override the default stylesheet in favor of your own box styling, if you’re not a fan of the default.