If you’ve upgraded to Mountain Lion, you might have encountered GateKeeper, Apple’s new security scheme to protect you from malicious apps. In short, GateKeeper expects applications to be “signed” with a security certificate it recognizes and, if not, will keep you from opening the application.
The problem is, a lot of older apps that are otherwise harmless were not signed with an Apple certificate, but the warning makes it seem as if they are problematic anyway. To get around this, you either change the setting in System Preferences to disable the protection entirely (that just takes you back to the way things were before Mountain Lion), or you can get around it on a case-by-case basis.
If you double-click an app that gives you that warning and prevents you from launching it, right-click (or control-click) on the application instead and choose “Open” from the menu that appears. You will still get a warning, but this time there will be a button that will let you launch the app anyway. After that, it won’t ask you again for that application.
To disable the setting entirely, go to System Preferences -> Security & Privacy, and click the General tab. Click the lock at the bottom, left to unlock this pane, then choose Anywhere.