MSDN subscribers: http://www.microsoft.com/windowsazure/msdn-benefits/
Non MSDN subscribers for the 90 day trial: http://www.microsoft.com/windowsazure/free-trial/
There used to be a link on the MSDN subscription page but it has seemed to have been moved so I wrote this blog post so I never lose it again and hopefully inform others.
The benefits vary depending on your level of MSDN subscription. For example ultimate users get 1500 compute hours per month - I think that is pretty good.