As with all providers, the HTML editor provider that the core and many modules use can be changed by altering the following web.config value

<htmlEditor defaultProvider="FckHtmlEditorProvider">

For more information on changing the HTML Editor Provider visit the Modifying the HTML Editor Provider entryModifying the HTML Editor Provider entry.

The defaultProvider value can be changed dependant on the available html provider. Over the lifecycle of DotNetNuke there have been a number of default providers

Versions 2.0.0-3.0.0
Ftb3HtmlEditorProvider - based on the freetextbox project (Freetextbox)

Versions 3.0.0 - 5.3.3
FckHtmlEditorProvider - based on the FCK editor (FCK Editor)

Versions 5.4.0 - 6.2.0
TelerikEditorProvider- based on the telerik radeditor (Telerik radeditor)

Versions 6.2.0 - ?
RadEditorProvider - Improved Telerik editor provider contributed by DMX (Telerik radeditor)

Version 9.2

DNNConnect.CKE - you'll probably want to update this one to something better, like the latest CKEditor

Alternative html providers

Cutesoft - Cutesoft

Wikiplex - Wikiplex

RADEditor - RadEditor