The plugin I am going to cover in this post, is an existing one which has been standardised; BBCode.
This plugin allows the use of bbcode to format posts and pages rather than HTML. BBCode may not be as popular as it was, but it is a fairly friendly way of formatting text.
Standard bbcodes are supported (full list of supported bbcodes can be found here).
You can combine this plugin with Shortcodes in Comments to allow your readers to use BBCodes when they leave a comment.
The plugin can be downloaded via my Development site.