Apple - How can I open a single app by an unidentified developer without allowing all

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.

To enable the execution of an app against the policy that you have set in System Preferences, then you can simply right click on the app in question, and click open. This will produce a further "Are you sure?" check, but will allow you to open an app and over-rule your app signature settings without you needing to go into System Preferences each time.

Although Ctrl+Right Click might work for some people, it didn't for me. They key to fix it is open Terminal.app (under /Applications/Utilities) and type in

xattr -d com.apple.quarantine

then at the end press space and drag the application you are trying to open in there, so it should look like this (this is an example for Google Chrome):

xattr -d com.apple.quarantine /Users/username/Desktop/Google\ Chrome.app

(here username refers to the user account name)