Unsanity has released Paranoid Android to protect Mac users from a security vulnerability in OS X.
This weakness potentially allows a malicious hacker to execute arbitrary commands on their Mac, such as deleting a home directory or other damaging moves. The weakness exploits parts of a URL that define what application should be used to handle that address.
The software exists to protect users against the weakness (until Apple releases an official fix, the developers say). It does this by watching the URL schemes that are requested and delaying them until you've had a chance to say whether you'd like to proceed or not.
Apple has been notified of the vulnerability. Paranoid Android is free.