Easy Vignette (and other effects) in Adobe After Effects

http://www.estrellastudios.com/blog/blog-video/after-effects-tip-quick-vignettes/

Here's a good tutorial. Basically, you create a layer mask above all other clips, double click the ellipse tool (which creates an ellipse that just fits inside your video), and expand the Mask settings and alter them (increase feather to 180, expansion down to -40). Then you can add an effect to get darker corners-- like Exposure, and set it to, say, -1.50. Of course you can add other effects too such as blurring.