For a hosted web application, what is best way to implement user registration and authentication?

Question by Raghu: For a hosted web application, what is best way to implement user registration and authentication?

This is an internaet application developed using JSP/Servlets. I plan to use any of the popular hosting services. I need to have framework for user registration, user authentication, role based authentication. What are my options to implement this? Who are good hosting services available?

Best answer:

Answer by creative_artistguy
Use Sentrylogin.com, by far the widest usage, and easiest to deploy. Supports IPN.

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