active_page.py

from django.template import Library
register = Library()

@register.simple_tag(takes_context=True)
def active_page(context, page_name):
    active = active_class(context, page_name)
    return ' class="%s" ' % active if active else active

@register.simple_tag(takes_context=True)
def active_class(context, page_name):
    if context.get("current_page") == page_name:
        return 'active'
    else:
        return ''

base.html

<ul>
  <li {% active_page 'register' %}><a href="{% url 'register' %}">Register</a></li>
  ...
</li>

urls.py

from django.conf.urls.defaults import patterns, url

urlpatterns = patterns('app.views', 
    url(r'^register/$', 'register', {
        'template_name': 'appregister.html',
        'extra_context': {'current_page': 'register'},
    }, name='register'),
)

views.py

from django.shortcuts import render

def register(request, template_name=None, extra_context={}):

    form = ...

    return render(request, template_name, dict(
        form=form,
        **extra_context
    ))