Getting namespace prefix of an org

Query for it, using a class that you know will be there:

ApexClass myClass = [
    select NamespacePrefix
    from ApexClass
    where Name = 'SomeClassNameHere'
];

Spotted in the Spring `16 release notes under Changed Objects:

Organization

The following new fields have been added:

• NamespacePrefix
• SignupCountryIsoCode—Represents the two-character ISO country code specified by the user for a sign-up request.

It appears it will now be possible to directly query for the NamespacePrefix without needing to have a known Apex Class or Trigger present.