Disable “Downloaded from Internet” Warnings on Mac OS X’s

Using the Terminal, Snow Leopard (10.6) users can enter this command, substituting “~/Downloads” (the Downloads folder inside the user folder) for whatever file or folder where they’d like to prevent future download warnings.

In 10.6, copy and paste this command:

xattr -d -r com.apple.quarantine ~/Downloads

To remove the warning dialog from already-downloaded files, you need to use Terminal (in Applications -> Utilities).
In 10.5, copy and paste this command:

find ~/Downloads/geeklog-1.6.1 -type f -exec xattr -d com.apple.quarantine {} \;

Note that these commands can take a while to run if there are a lot of files in your Downloads folder. You can also change the directory they run on by modifying the ~/Downloads bit of the command—just change it to reflect the full path to whatever folder you’d like to update. (Remember to use backslashes before spaces, if any of the items on the path contain spaces.)

Permanently disable the warning system

If you’d like to disable the warning dialogs completely (for any files you download in the future), open Terminal and copy and paste this command:

defaults write com.apple.LaunchServices LSQuarantine -bool NO

After you get the command prompt back (you won’t see any feedback; the command prompt will simply appear again), restart your Mac. From now on, you won’t be warned when opening downloaded files. It goes without saying, but making this change reduces the security of your machine.

If you do permanently disable the system, and then decide you’d like it back, repeat the above command, but change NO to YES and reboot your Mac.