Yes you can, with a Mighty Mouse
You can open any application with one click.
If you have a Mighty Mouse, of course.
Just set the track wheel to Other, which lets you select any application to be opened when you press the track wheel button.
Obviously this is helpful if you want to quickly access just one application you use frequently (in my case I have set the button to launch Safari).
For accessing --any-- application I can't think of anything else than dragging it into the Dock.