I use Untappd, and Next Glass. Coincidentally, the latter apparently recently bought the former, so some of their functionality might soon be joining together.
Untappd is a "social" app. You rate beers, can attach photos and tag a location to your rating, and can "friend" other people and comment on their check-ins, etc. That part is too "Facebook-y" for my taste, but I like the fact that you can search for tons of beers, see how well-regarded they are by a large user base, see what beers are available near you, which breweries are near you (nice when traveling), etc. It also gives recommendations of local beers based on the beers that you rate highly, and your current location.
Next Glass is to beer & wine as Pandora is to music. They take beers and analyze their chemical makeup. When you rate beers (and wines), it will give you recommendations of other beers you might like based on having a similar composition, with a predicted numerical rating of how much you'll like them. The downside is that their database of tested beers is smaller than what you'd see at a typical user rating site BeerAdvocate, RateBeer or Untappd.
I've tried BeerMapper, but the interface is clunky and I don't think it does a good job at all of making recommendations.
Not an app, but one cool website I like is
http://www.beergraphs.com. It uses the raw data from Untappd, applies its own rating system, and allows you to search for beers by style, brewery, city, state, region, and make comparisons within style, etc. The search page is here:
http://www.beergraphs.com/leaderboards/