Eucalyptus is an open-source software for building private and hybrid clouds. It implements the Amazon Web Services API and allows users to deploy and access cloud resources. Eucalyptus has a modular architecture with controllers that manage user requests, virtual networking, and hypervisor-level virtual machine execution across physical cluster nodes. While still in development, initial experiments show Eucalyptus can provide cloud-like functionality and performance on small Linux clusters. Key challenges include improving extensibility, networking support across clusters, security, and easing installation and maintenance.