You can create DITA source files with a plain text editor, for example Microsoft Notepad for Windows. Editors at varying levels of "DITA-awareness" are also available, both free and for purchase. These editors help you create DITA documents that are well-formed and valid.
Some well-known DITA-aware editors are Adobe FrameMaker, Altova XMLSpy, Arbortext Editor, justsystems XMetaL (DITA edition), Pixware XMLmind, Stylus Studio, SyncRO Soft <Oxygen/>, and Syntext Serna.
Authoring tools are not part of DITA Open Toolkit or any of the prerequisite tools for the Toolkit.
Some of these DITA authoring products contain embedded copies of the DITA Open Toolkit and hence you will not need to install the Toolkit separately.