It's a third person online multiplayer brawler with a premise that makes no sense whatsoever. You can play as a baseball player, football player, boxer, firefighter, or rock star and beat the crap out of other players in hectic team battles up to 8 players. The game lacks a severe amount of content, has bad "engrish" translations all over the place, weird voiceovers, and horrible lag issues due to Peer-to-Peer connections...
BUT GOD DAMN... THIS GAME IS FREAKING FUN.
It's the most fun I've had playing a free game in a while. The gameplay is fairly simply, but strangely satisfying. All the classes are fairly balanced, too. With the game's wacky premise, there's so many ways they can go from here. They could add more wacky classes based on sports and professions like basketball player and hockey players, or doctors and artists, or something like that. Then they could add game modes like soccer, capture the flag, and what not. The sky is pretty much the limit for this game.
Plan B is quickly gaining popularity through word of mouth. It's currently in a global beta test so anyone can play (The community is mostly filled with Koreans, Russians, and Americans.) Try it out. Also be sure to play with people around your area so lag will be minimal.
EDIT (03/29/2010): The beta test for Plan B is now over. Now we must play the waiting game.