How do I set my organization up with a standard email signature block?

There is no default signature at the Organization level, only the user level.  However, if you are on the Enterprise edition you can create a scheduled workflow rule that will run on all users and put in a default signature along with a workflow rule that will set a default signature for new users.  You can even put dynamic elements in the rule such as '$FirstName' and '$LastName', along with contact information such as phone numbers if it exists on the user record.  I would recommend hopping on one of our Advanced Q&A webinars to go into the specifics of creating these rules (http://webinars.clarizen.com).