Untappd

Untappd (notice the lack of an 'e'; it takes a little while to get used to, but now, if I ever have to use the actual word, I'll probably misspell it) is the ultimate intersection of beer and social media.  It allows you to log and rate beers, and connect with friends.  They have apps for Android and iOS, as well as regular and mobile websites.

Untappd has a very large database of beers, and I've found it very uncommon to find beers that aren't there; however, if you do find yourself in that situation, it's fairly easy to add the beer yourself.  Once you've found a beer, you can check it in, add it to your wishlist, or find nearby places where others have checked it in.  When you check a beer in, you can also add a rating, a location (provided by Foursquare), a picture, and a 140 character comment.  Once you've checked a beer in, Untappd will give you recommendations of other beers you might like.  You can also view the check ins of your friends, and other nearby users.

Here are the Beer Barons on Untappd:
The Big Guy
El Duque
Knight
O-Dawg
Professor Beer

There are a few alternatives out there, the most legitimate of which seem to be Beerby (rhymes with 'nearby') and Pintley.  I haven't been using either for very long, but it seems that Beerby is a less complete version of Untappd; at one time they were competitive, but Untappd seems significantly more robust at this point (much of which is due to Untappd aquiring RedPint).  Also, (on Android, at least) the Untappd app is more polished.  Pintley isn't exactly analagous, and seems more geared towards generating recomendations, which I haven't found terribly usefull (though I have been using it for a very short time).  I plan to continue using all three for a while (with Untappd as my preference, if convenience is an issue), and if either of the other apps proves to be more usefull than my original impression indicates, I will be sure to let you know.