Django
Official site
http://www.djangoproject.com/
Comments:
Great if you want a Python-based full stack framework.
Works less well if you don't always use an RDBMS
Very strong form handling capabilities
Has many pluggable open source applications
Start-ups using:
Playfire
Smarkets
IvyLees
BonjourParis
Urban Alarm
Clevertim CRM