You probably heard about a good WYSIWYG editor called Imperavi Redactor or even tried it. If you like it, there's a good news for you. Our excellent community funded buying Redactor OEM license so now it can be used for free for any Yii-based product.
Since we do not want to bloat Yii itself too much, it will not be integrated into the core framework. You can get it as an extension from YiiExt repository.
Note that Redactor itself isn't OpenSource product so check if license is OK for you before actually using it.