I'm a fan of Django, so my ideal stack looks something like this:

  * puppet - managing server packages / infrastructure
  * monit - monitoring server processes / fixing things
  * django - primary web framework and ORM
  * amazon mysql - it's hosted, and works via plug-ins with Django
  * amazon s3 - storing static assets (images, css, javascript, etc.)
  * amazon elastic load balancer - for scaling incoming HTTP requests across multiple web app servers
  * amazon autoscale - for spinning up new web app servers to handle spikes in traffic
  * rabbitmq - message queueing
  * celery - processing async tasks in a robust fashion. must have
  * memcahed - no explanation necessary
  * git
  * fabric for deploying software
  * jenkins for testing / building software
  * nginx for buffering elastic load balancing requests to web app servers