When it comes to customizing your textarea box the two most popular choices are TinyMCE or Openwysiwyg.  I wish I could say this is a close battle between two really good options.  The truth is that there is only one viable option to choose and that is TinyMCE.

The problem with Openwysiwyg is that it addeds extra markup for no reason and does not adhere to web standards.  Just try using their demo and look at the code it creates.  It makes you want to vomit.

On the other hand, TinyMCE is more modern and adheres to webstandards.  Check out the code in this blog post, compliments of TinyMCE.