Real estate web site Homethinking compares neighborhoods in different cities to help you find a new neighborhood you'll like. Say, for example, you want to leave New York for Los Angeles. Homethinking tells you which neighborhoods in LA might fit your tastes based on which neighborhoods you like in New York (e.g., East Villagers might like Los Feliz or Echo Park).
For a more detailed look at how the neighborhoods match up in the categories of culture, people and activities, you can click on a specific neighborhood.
I personally don't know how well this works. I tried to compare the city I grew up in, Miami, with the city I now live in, Atlanta, but most of the neighborhoods I love in Miami are missing or miscategorized (by my standards) and Atlanta isn't even on the list. Why don't you Angelenos and Manhattanites give this Web site a spin and let us know how well it matched your expectations?
And whenever or wherever you do decide to move, be a good Unplgg'r and pack up your stuff with the digital message tape.