Yahoo! Mail's rich text editor in the composition form is clean and provides the core set of formatting options you'd need. The implementation is simple and intuitive. A few things they do differently include: rolling some commonly visible controls into drop down menus, e.g. text alignment and list style blocks.

1. Font face selection and font size are in HTML form selects.
2. Font color selection. Highlight or background color uses the same color selection control. There seems to be a bug here, where Italics is shown toggled on when the selected text is not italicized.
3. I guess Yahoo! users like emoticons. :)
4. Link creation is handled via a JavaScript dialog.
5. Text alignment is rolled into a single drop down.
6. List style in a single drop down.
7. The message composition form in the context of the full page.