Example: Content negotiation in Django. This may be more complicated than it needs to be; it'd be simple to write a library which deals with the common cases rather than having to do the full broker registration.

views.py

from broker import Broker  # https://github.com/zacharyvoase/broker
from django.core import serializers  # https://docs.djangoproject.com/en/dev/topics/serialization/
from django.http import HttpResponse
from django.shortcuts import render

from myapp.models import Thing


# This is just a library I wrote once to help do content negotiation.
# We define a 'renderers' for each supported content type (HTML and JSON)
# and then pass the list of things into that.
things_renderer = Broker()

@things_renderer.register('text/html')
def render_things_html(request, things):
    return render(request, 'things/list.html', {'things': things})    

@things_renderer.register('application/json')
def render_things_json(request, things):
    return HttpResponse(serializers.serialize('json', things),
                        content_type='application/json')


# The view itself:
def list_things(request):
    things = Thing.objects.order_by('-created')[:20]
    return things_renderer(request.META['HTTP_ACCEPT'], request, things)