Typical beer has around 4 to 6% ABV (alcohol by volume); particularly strong beers using special strains of yeast and careful brewing can get up around 12%, giving quite a surprise to the unwary drinker.
In the 1990s, Samuel Adams upped the ante with their 17.5% ABV Triple Bock (1994), 21% Millennium (1999), and 24-27% Utopia series (2002-2007): beers said to be more like brandy or port than traditional beers, but still made without distillation. (No, I haven't tried any of these yet. You buying?)
Then, just over a year ago, interest in the century-old technique of "ice distillation" heated up. Ice distillation takes advantage of the fact that water freezes more easily than alcohol: freeze beer just right, and you can remove much of the water (as ice) and get a stronger beer left behind. (Note that according to the wik, freeze distillation can also concentrate poisonous compounds like fusel alcohols; so don't try this at home.)
Breweries using this technique have been in an arms race recently, rapidly taking the record from 31% ABV to an astounding 43% -- 86 proof, the same alcohol content Jack Daniels whiskey used to have (before JD's wimp-out of a few years ago).
"I am confident we can get to 50% with all the right qualities,", says Georg Tscheuschner of Schorschbräu, the maker of the 43% "Schorschbräu Schorschbock".