XTypeS is now superseded by Xsemantics!
Xtext is a Language Development Framework based on Eclipse. With Xtext you can easily create your own programming languages and domain-specific languages (DSLs).
XTypeS (Xtext TypeSystem) is a DSL (implemented in Xtext itself) for writing type systems for languages implemented in Xtext. It then generates Java code that can be used in your language implemented in Xtext for scoping and validation (XTypeS also generates a validator in Java).
XTypeS is released under EPL (Eclipse Public License).