Obscuring a face in After Effects

@AdobeAE notes a tutorial from AE CS5: Learn By Video on obscuring a face in After Effects by isolating it with Roto Brush & selectively applying a Mosaic effect (embedded below). There's more on the Roto Brush in AE Help and in posts tagged roto.

For other ways to do the same thing, and with earlier versions of AE, see for example Bob Donlon's The Mask of Shame and Obscure a logo, a face, or a license plate in After Effects at Toolfarm. Another example clip from AE CS5: Learn By Video shows a related technique-- how to combine motion tracking and rotoscoping to isolate and selectively color-correct an actor's face: