http://alice.unibo.it/xwiki/bin/view/Tuprolog/ (LGPL)

tuProlog is a light-weight Prolog system for distributed applications and infrastructures, intentionally designed around a minimal core (containing only the most essential properties of a Prolog engine), to be later configured by (statically and dynamically) loading/unloading libraries of predicates. tuProlog also natively supports multi-paradigm programming, providing a clean, seamless integration model between Prolog and mainstream object-oriented languages -- namely Java, for tuProlog Java version, and any .NET-based language (C#, F#..), for tuProlog .NET version. It is also easily deployable, just requiring the presence of a Java/CLR virtual machine and an invocation upon a single self-contained archive file. Interoperability is further developed along the two main lines of Internet standard patterns and coordination models.