Recently I came across a new contributed module of Drupal (released Dec 1st 08) that makes it even easier to include a WYSIWYG-editor of your choice. The WYSIWYG API fully integrates with Drupal's Input Formats, which makes a really nice tool. WYSIWYG API makes it possible to show different supported WYSIWYG-editors to different roles on different selected input formats. Crazy!

Here's how it works:

1. Download WYSIWYG API modules from http://drupal.org/project/wysiwyg and activate it at admin/build/modules

2. Download any of the supported WYSIWYG-editors. Currently you can use:
   TinyMCE 2.x, TinyMCE 3.x, jWYSIWYG 0.5, NicEdit 0.9.0, FCKeditor 2.6.x, Whizzywig 55+
   (see the official project page for more recent information and links to those editors)
3. Upload the WYSIWYG-editor of your choice inside the WYSIWYG API modules folder.
   Be carefull not to use the /editors subfolder, which is not meant to include the editors themselves, but additional scripts for API integration of each editor. See this Screenshot for an example of TinyMCE inside the WYSIWYG module's directory.
   
   (The correct directory for each supported editor is also available at admin/settings/wysiwyg/profile)
4. Navigate to admin/settings/wysiwyg/profile and assign your editor to suitable input formats.
5. Don't miss to assign suitable permissions at admin/user/permissions if you want specific roles to be able to use your editor.
6. Configure your editor of choice using the "edit" links at admin/settings/wysiwyg/profile.