Amazon web services recently introduced a free tier, where you basically get free stuff to try out AWS and run tiny sites and projects. Basically it's free as long as you remain below a certain limit of bandwidth, disk storage etc.
Since going over the limits can quickly become quite expensive (for a hobbyist) I would like some recommendations or suggestions about which AMI's I can run on the free tier, for the purpose of trying out Ruby on Rails and/or Django.
Use the Amazon Linux AMI. It's the only AMI that's officially supported and maintained by Amazon. It's optimized for EC2 with the ec2-api-tools included, boots from EBS, and a package repository that's hosted on EC2. It also includes great features like CloudInit.
There's more info info in the Amazon Linux AMI User Guide.
For anyone who finds this via Google, try the official Ubuntu AMIs that work: http://cloud.ubuntu.com/2010/11/using-ubuntu-images-on-aws-free-tier/
You might want to check out ami-c2a255ab. Its a 10GB EBS-based Ubuntu Lucid server. You will have to manually install what you need but it seems to be the only 10GB Ubuntu server right now.
Note if you're using Amazon's web panel, when you open the "Launch Instance" dialog you can filter the list of AMIs by "Free Tier Eligible", and the CLI has a filter param as well. That was the quickest way I found to see what AMIs are available for the free tier micro-instances.
You can get Ruby on Rails, Django and many other EBS AMIs from our project BitNami The EBS are completely free and we try to keep them up to date