DefaultApp is a MacOS 8 utility that allows you to open documents matching certain file/creator types with the applications you specify, instead of the applications that created those documents.
For instance, DefaultApp can be configured so that when you double-click a SimpleText document in the Finder, it gets opened in a more powerful text editor, like Style or Tex-Edit Plus. Or you can have Script Editor compiled scripts opened with Script Debugger. Or Internet Explorer documents opened with HTML Viewer. Well, you get the idea.
Default App is a beta version, although usable, it will undoubtly contain bugs, some of which you will not notice. The final release version will be posted here as soon as it is released.