How about Google App Engine for startups? They give a free tier, which I presume is free for life as long as the web application stays within quota.
Ofcourse there is lock-in, there is no free lunch anywhere! But it should be ok for startups with no capital, who just want to figure out their business model. Once the idea is validated, they can always learn from the experience, and rewrite the application if platform absolutely needs to be changed.
Disclaimer: I don't get paid by Google for writing this, though I am trying to do something with google app engine (python) and that may lead to some bias on my side :-)