f_underscore.js is a library of functions that create and compose iterator functions. It is designed to be used in conjunction with underscore.js's functions that employ iterators, functions like filter and map. With the f_underscore.js library you can wield and weld iterator functions declaratively and dynamically. It's the tie clip to go along with underscore.js's tie.
Documentation can be found at http://krisjordan.github.com/f_underscore/. Annotated source is available, as is a qunit test suite. The project is hosted on GitHub and available under the MIT license.