Linguistic Knowledge Builder (LKB) is a free and open source grammar engineering environment for creating grammars and lexicons of natural languages. Any unification-based grammar can be implemented, but LKB is typically used for grammars with typed feature structures such as HPSG.
It is implemented in Common Lisp, and constitutes one core component of the DELPH-IN collaboration.
References
LKB Wikipedia(Text) CC BY-SA