I was so excited when I downloaded the Vignette application for my Android phone. In fact the main reason I chose an Android phone was to use photography apps, and Vignette is the best one I've found so far. It even inspired me to start my own Phone Project 365 using photos from my phone camera only. There are so many effects you can use, but my favourites so far are:
Toy Camera Black and White
And of course, the Vignette effect itself
For a little over £2 I'd say this was more than worth it!