NOTE. The functionality of the Default Text Editor plugin has been implemented in the Eclipse Platform with the Neon release. Check the release notes for details.
Eclipse users usually work with many different file types. Some of the file types may be opened by default in an external editor instead of in the Eclipse workbench. This happens if Eclipse has no editor available to handle that particular file type, but there is one installed in the operating system. In such case Eclipse assumes that it is better for the user to have the file opened in the external system editor.
Lots of users are quite annoyed by this behavior, especially when it comes to text-based files. They would prefer to have the file opened in the plain text editor of Eclipse instead of switching the context to the external program. Unfortunately, there is no easy way to change this in the preference settings. It's possible to associate a specific file extension with the plain text editor, but this must be done separately for every file extensions. There is no way to say "all text files of unknown type should open in the text editor".
Here comes the Default Text Editor plugin. It takes advantage of the Editor Association Override extension point introduced in Eclipse 3.8. When the plugin is installed it will change the default behavior of Eclipse and will opened all text files of unknown type in the plain text editor. Binary files like images may still be opened in an external system editor. As simple as that.
The plugin is available on the Eclipse Marketplace. It can also be installed through an update site. More info is available on the GitHub project.