(This article was first published on dylan's blog, and kindly contributed to R-bloggers)
Hamish Bowman recently posted a new GRASS module (d.barb) that can be used to depict the direction and magnitude components of some vector (e.g. wind field) along a raster surface or at points in space. An example (c/o Hamish):

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Zero Inflated Models and Generalized Linear Mixed Models with R.
Zuur, Saveliev, Ieno (2012).