How can I open a single app by an unidentified developer without allowing all I know to secure safety it is now not allowed anymore to open apps that were not downloaded from the mac app store.

But surely there is a workaround for this to open an app that you know is 'good' but just isn't in the app store.
Preferably allowing only that app and not allowing all downloaded apps to be opened.



How do I open an app that is not downloaded from the app store in mountain lion?
You can ignore the gatekeeper restrictions on a per-app basis by right/control-clicking and selecting "Open". And because the check is done only on the first open, you can run it normally afterwards.

More details in: 


John Siracusa's review of Mountain Lion
Apple article HT5290, Mountain Lion: About Gatekeeper.