Hi Robert,
Yes, there are various ways through menus and keyboard shortcuts to launch a program or open an item without double clicking on it, but that wasn't the OP's question. They want to be able to do that by clicking on the item once without doing anything else.
The Finder in OS X will let you do the same thing OS 9 did. Highlight the item by clicking on it once and then press Command+O. Same as OS 9. If it's an app, it'll launch. If it's a file of some sort, it will launch the associated app and then open in that app. You can also add anything to the Dock in OS X. Once there, it's a one click launch.
A double-click, however, should never be
required.
It needs to be a requirement. How else would the system know whether you were clicking, say on a Photoshop document, once just to highlight it or to open it? If everything required only once click, you would
constantly be opening files or launching applications you didn't intend to. There has to be a say to separate the meaning of your actions without always having to resort to a second action after first clicking on an item in order to do something with it.