MovableType Open Source has a nice rich text editor, but pasting media embedding code directly into it doesn't work. In my experience working with it, it does work if I switch the editor to "raw code" first.
To do so, I click on the <A> button, as pictured here:
Once the code has been pasted, clicking on the underlined A will return to rich text mode.
NOTE: I'm no longer using the rich text editor. It worked perfectly for me on Mozilla powered browsers like Firefox and Iceweasel, but not with WebKit powered browsers like Safari, Midori, or Chrome. Nowadays I'm using the markdown (a simplified text markup syntax, see this) editor and I haven't looked back.
