I’m not going to give a full run down of what Eucalyptus is but just point you to their marketing material at their website. The quick summary is it offers the Amazon Web Services loaded on a local computer. These include several of the most interesting services: EC2, S3, EBS, AMI, IAM, and recently they added Autoscaling, Elastic Load Balancer, and Cloudwatch. If that alphabet soup has your interest piqued, then you should continue reading.
Building one of these using their pre-packaged images is dead simple. I’m not one to do anything the simple way and decided to build everything from the source directly from their Github repository. This was not an easy task but definitely taught me a lot about their software and the components of the system. I would recommend a first-time user to not take my route and just take their binary builds from RPM or their ISO image. Fedora Core has these as well and the guy who supports it is a great guy. Please take the path of least resistance first to get familiar with the software.