Traduki is a suite of open source linguistic software. Originally intended to be "just" a machine translation software, Traduki got its author so involved that it eventually grew into a much larger scale project.
Moses is a statistical machine translation system that allows you to automatically train translation models for any language pair. All you need is a collection of translated texts (parallel corpus).
The Translate Toolkit is designed by localisers for localisers. Its aim is to make localisation easier and of higher quality. The toolkit can convert between various different translation formats and makes it possible to stay in one format across all localisation tasks.
Apertium is a machine translation platform, initially aimed at related-language pairs, but recently expanded to deal with more divergent language pairs (such as English-Catalan). The platform provides a language-independent machine translation engine, tools to manage the linguistic data necessary to build a machine translation system for a given language pair, and linguistic data for 21 language pairs, with more under development.
The platform also includes cell phone applications, blog plugins, web services and several graphical user interfaces. Apertium may translate plain text, web sites or documents without loss of formatting.
The Okapi Framework is a set of interface specifications, format definitions, components and applications that provides an environment to build interoperable tools for the different steps of the translation and localization process.
Project Open is a general-purpose project management system with a translation module. The source license is a GPL hybrid, and the translation module is FL (“Free License”), which is “pseudo FOSS”, with extensive limits on redistribution of code.