BitNami RubyStack provides a fast, easy way to develop and deploy Ruby on Rails applications. It includes Ruby, Subversion, MySQL, SQLite, ImageMagick, and several Ruby Gems, and will optionally install Apache 2.2 with rewrite and proxy support. It supports Windows, Linux, and OS X, so you can share the same Rails environment on multiple platforms.
Puppet lets you centrally manage every important aspect of your system using a cross-platform specification language that manages all the separate elements normally aggregated in different files, including users, cron jobs, and hosts, along with obviously discrete elements like packages, services, and files. Its simple declarative specification language provides powerful classing abilities for drawing out the similarities between hosts while allowing them to be as specific as necessary, and it handles dependency and prerequisite relationships between objects clearly and explicitly.
Elektra is a universal hierarchical configuration store, similar to GConf and the Windows Registry. It allows programs to read and save their configurations with a consistent API, and allows them to be aware of other applications' configurations, leveraging easy application integration. While architecturally similar to other OS registries, Elektra does not have most of the problems found in those implementations. Elektra includes a library, an API, and commandline and GUI tools for administration tasks.
Virt-Factory is a framework for managing a large number of Xen virtual machines, such as might be found in data centers or large lab configurations. It works by managing a pool of registered physical hosts and then deploying virtual machines, which have specific roles as defined by their "profiles" (profiles act as shareable virtual appliance definitions), onto those hosts. Virt-Factory provides both a Web UI (for ease of use) and an XML-RPC API (for scripting of admin actions). A command line interface (along with other advanced batch management features) is coming soon.