FckEditor 2 used to be the most popular richtext editor until 2010, when it got replaced by CkEditor 3 (version 4 coming soon). The editor offers a number of configurations and extension points, for details, please check out its
web site.
DNN used to incorporate FckEditor 2 as default editor in DNN 4 to 5.4, using a provider written by Locopon aka Mauricio Marquez.
Due to security issues, it is strongly suggested to upgrade either to RadEditor provider (included with DNN 6) or CkEditor 3 provider, created and very well maintained by WatchersNet (available for free download from
here).