GooglePlus
Google is well known for adding fun, holiday themed effects to Google+. Their Hoff April Fools day prank was in a word: Awesome, now they’re gearing up for Halloween, and whether you want to celebrate Halloween or not there’s a spooky or fun new way to celebrate it with Google+.

When logging in to view your photos on Google+ you can now add either a Spooky or Fun effect to your uploaded photos.
Halloweenify

The options are automatically added and are pretty cool. Check out what they did to my photos :

Fun Effect
Fun Effect
Spooky Effect
Spooky Effect

The fun will last over Halloween, which is on the 31st of October, but no telling how long it will last after that, so get in quickly.