From http://www.twelf.org, heavily edited:

    Twelf is a research project concerned with the design,
    implementation, and application of logical frameworks.  It
    provides a uniform meta-language for specifying, implementing, and
    proving properties of programming languages and logics.

    Example suites include Cartesian Closed Categories and
    lambda-calculus, the Church-Rosser theorem for the untyped
    lambda-calculus, Mini-ML including type preservation and
    compilation, cut elimination, theory of logic programming, and
    Hilbert's deduction theorem.

    The principal authors of Twelf are Frank Pfenning and Carsten
    Schuermann, with major contrubtions by Brigitte Pientka, Roberto
    Virga, and Kevin Watkins.