Sensarea is a free video editor which allows you to select, track and modify objects.

You could use the program to blur or pixelate someone's face throughout a clip, for instance. This doesn't have to involve a lot of tedious frame-by-frame editing: at a minimum you can just select an area, and an effect, then have Sensarea track and process that object for the rest of the movie.

Setting this up is much like working with selection tools on a graphics editor. You're able to draw a rectangle or an ellipse, use a magic wand-like colour selection, maybe paint an area, then dilate, erode, move or rotate the mask to produce the results you need.

Effects on offer include Hide, Blur, Pixelize, Zoom, Negative, various colour adjustments, and more.

The end results can be exported as an AVI video, a sequence of images or an SWF.

Help is minimal, but the program isn't difficult to use, and there are video tutorials around which will show you the basics (see this explanation of how to achieve "Blurred faces in video").