OpenStack: This open source challenger to Amazon follows the Amazon cloud services model (EC2 plus S3) and provides an Amazon-compatible API to compute and storage services, using a dashboard that gives administrators control over cloud resources while providing end-users an easy self-service portal for provisioning their VMs and storage
CloudStack: Well-positioned for use by service providers, CloudStack features a great Web UI for administering cloud resources, a brandable end-user interface, and the concept of domain admins, who sit between the admins and the end-users, making it suitable for use by resellers. CloudStack provides an Amazon AWS compatible API with full support from Citrix.
Eucalyptus: A logical choice for organizations that need to run public/private hybrid cloud services, and its integration with the RightScale myCloud service gives administrators a single tool to control this new hybrid cloud setup.
Ganeti: A management tool for small clusters of virtualization servers running Xen or KVM that supports setting up instances using a file, an LVM volume, or a DRBD mirror. Ganeti can do live migrations of instances from one server to another, while providing real HA fail-over versus rebooting lost VMs elsewhere on the cluster. Only local storage is supported, however.
Open v Switch: An open source distributed virtual switch that rivals those in vSphere 5 and Hyper-V 2012, Open vSwitch is used in Citrix XenServer and the Xen Cloud Platform, supporting Xen, KVM, and VirtualBox. Advanced flow monitoring, vprobes, spans, QoS, and the ability to deploy as a virtual or physical appliance, give Open vSwitch a features list more impressive than some hardware switches
Cloud Foundry: Supports a wide range of services and integrates with many clouds -- vSphere/vCloud, Amazon Web Services, OpenStack, Rackspace, Ubuntu, and more -- running on private or public infrastructure. Developers can quickly build next-generation applications by writing once locally, testing, scaling, and deploying with no code changes.
JBoss AS 7: Revisions in JBoss AS 7 return the solution to contention, unlike its predecessor, AS 5, writes Andrew Oliver, adding that "it's free and it doesn't suck."
Jclouds: A cloud services API abstraction layer, designed to make dealing with multiple cloud services simple and portable and mired in as few dependencies as possible
Puppet: A solution for administrators of heterogeneous networks that run various OSes who requie a single tool for their management
Chef: A Ruby-based solution for configuration management power with "recipes" that describe a node's packages, configurations, and services; applicable to either a single machine or a large cloud deployments of more than a couple hundred nodes
Juju: A tool for Ubuntu cloud management featuring a reusable set of resources to bootstrap cloud services and even the servers they're running on; combines metadata for describing services and scripts (aka Charms) for controlling them; delivers consistent, reproducible service deployment, orchestration, and scaling
GlusterFS: A distributed file system for commodity hardware that is monumentally simpler to install than GFS2 and a lethal enemy to expensive SANs.
Ceph: A distributed file system that is very well put together and offers everything you need, from basic network storage to high-performance clusters, for big data crunching.
FreeNAS: The open source FreeNAS storage platform with a long list of features but lacking support for WebDAV
Openfiler: Supports a variety of SAN and NAS technologies but, unlike FreeNAS, Openfiler is based on Linux and the LVM volume manager, and it contains support for WebDAV
NAS4Free: A spinoff and continuation of the FreeNAS 7 project with both fans and detractors