Umbraco uses TinyMCE for the rich text editor data type (the same JavaScript RTE as many other CMS’s including some of the Enterprise products).
In terms of customising the RTE, generally most of what’s in the TinyMCE Wiki will apply to Umbraco installations. However, there are also plenty of examples of Umbraco-specific customisation:
- Adding custom styling options to the ‘Styles’ drop-down:
Customised TinyMCE Editor to Allow Heading Tags to be applied
- Allowing users to insert their own micro-templated regions within the RTE:
Treasure Your Exceptions 