Since there were some significant changes in Sitecore's RTE when they upgraded to Telerik's new libraries from Sitecore 6.3 to 6.4. I'd written previously about adding buttons to the RTE before and was lookin for a good reason to write a supplemental article. Thankfully the new editor actually simplifies a bit of what you'll need to do from the earlier version of Sitecore.
I was recently asked by a content editor how to insert a YouTube video on a page in Sitecore and after thinking about the best approach to respond with my options were to tell the user to change to HTML mode in the Rich Text Editor (RTE) or for me to install one of the Shared Source Modules. I looked over YouTube Integration and as thoughtfully designed as it was I thought it would require too much effort for something I thought could be much simpler. I decided to extend the RTE with an additional button that opened a dialog window you could paste the IFrame embed code into.