In Django, can you add a method to querysets?

This is a complete solution that is known to work in Django 1.3, courtesy of Zach Smith and Ben.

class Entry(models.Model):
    objects = EntryManager() # don't forget this

    is_public = models.BooleanField()
    owner = models.ForeignKey(User)


class EntryManager(models.Manager):
    '''Use this class to define methods just on Entry.objects.'''
    def get_query_set(self):
        return EntryQuerySet(self.model)

    def __getattr__(self, name, *args):
        if name.startswith("_"): 
            raise AttributeError
        return getattr(self.get_query_set(), name, *args) 

    def get_stats(self):
        '''A sample custom Manager method.'''
        return { 'public_count': self.get_query_set().public().count() }


class EntryQuerySet(models.query.QuerySet):
    '''Use this class to define methods on queryset itself.'''
    def public(self):
        return self.filter(is_public=True)

    def by(self, owner):
        return self.filter(owner=owner)


stats = Entry.objects.get_stats()    
my_entries = Entry.objects.by(request.user).public()

Note: the get_query_set() method is now deprecated in Django 1.6; get_queryset() should be used instead in this case.

As of django 1.7, the ability to use a query set as a manager was added:

class PersonQuerySet(models.QuerySet):
    def authors(self):
        return self.filter(role='A')

    def editors(self):
        return self.filter(role='E')

class Person(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    role = models.CharField(max_length=1, choices=(('A', _('Author')),
                                                   ('E', _('Editor'))))
    people = PersonQuerySet.as_manager()

Resulting the following:

Person.people.authors(last_name='Dahl')

In addition, the ability to add custom lookups was also added.