Amazon announced yesterday that it is now offering Microsoft Windows Server 2008 R2 on the "AWS Free Usage Tier" for up to 750 hours per month. The Free Usage Tier includes:

Free Tier*

As part of AWS’s Free Usage Tier, new AWS customers can get started with Amazon EC2 for free. Upon sign-up, new AWS customers receive the following EC2 services each month for one year:

750 hours of EC2 running Linux/Unix Micro instance usage
750 hours of EC2 running Microsoft Windows Server Micro instance usage
750 hours of Elastic Load Balancing plus 15 GB data processing
30 GB of Amazon Elastic Block Storage (EBS) plus 2 million IOs and 1 GB snapshot storage
15 GB of bandwidth out aggregated across all AWS services
1 GB of Regional Data Transfer
I just signed up and you do need a credit card in case you go over those free limits, but other than that the signup process was easy and very quick (less than 5 minutes)