Hold Command(Apple key) and Option(alt) and click the Finder icon in the Dock...it'll hide all other apps. Press any other icon in the Dock and that will the the only showing app
ASM 2 (Application Switcher Menu) can do what you ask (hide all other applications except the Finder) and run in single application mode, with global settings and per-application exceptions. http://asm.vercruesse.de/
I use Keyboard Maestro. I created a key command that not only hides all open apps but also closes all finder windows so I have only my desktop in view. Get it at Version Tracker ($20).