There is a certain freedom of abandon that Halloween provides. It is a time of playfulness and wicked fun. Corporate web companies have also joined in on the fun as a way to strengthen customer relationships and become more personable and even a bit silly. Google has a history of changing it's corporate logo on holidays or commemorative events. Below are some samples of corporate playfulness / relationship-building design-gusto for the Halloween season.
Mailchimp 2009
Other examples of Halloween / Holiday corporate logos for the web.