WeaverBird is one of the first of the many independent plugins I think are going to be created for Grasshopper in the near future, it was created by Giulo Piacentino an architect with a Master of Science in Architecture from Delft University of Technology (very cool school) currently working with McNeel in Architecture training and support, innovation at Barcelona.
WeaverBird is a set of components that complement the existing mesh functions Grasshopper already has , I have played a little bit with it and I have to say its pretty fun it really powers up rhino to create parametric topological objects.

studies with Giulio Piacentino’s new ‘WeaverBird’ plugin for Rhino by Stuart Fingerhut

Description Straight from the site

This is an early release of WeaverBird (version 0.1.16), a plugin for topological transformations in Rhino 4.0 and Grasshopper 0.6.0043 or 0.6.0019.

Main topological commands:

- wbCatmullClark. Calculates the type of mesh-based recursive subdivision described by Edwin Catmull and Jim Clark, at first in their 1978 paper. The resulting mesh always consists of quad faces.

- wbFrame. Computes a new mesh with higher genus, where each face has a new hole in the center and resembles a picture frame. The resulting mesh always consists of quad faces.

- wbCarpet. Computes a new mesh with higher genus, where each face has a new hole in the center. The resulting mesh always consists of quad faces, and can be used to compute a Sierpinski carpet.

- wbWindow. Replaces each original mesh face with a new one, reconstructed on the inside. Each face has the same number of sides as the original one.

New mesh primitives definitions:

- wbPrism

- wbPyramid

- wbDiPyramid

Additional commands for tweaking:

- wbOptions. To set threading and other options.

- wbProperties. Opens a window with topological information about any mesh object in Rhino.