Like everyone else said. It's your duty to toast the couple at the wedding, help plan as necessary to make sure things go smoothly, make sure all the guys in the wedding party show up properly dressed, etc. You're the groom's champion. Just like in the old days when a man brought his "best man" with him to snatch the bride from her father's house.
As far as the party, you can help arrange it, pick the location, select the caterer, babes, make sure everyone pays their share, and make sure the groom doesn't do anything he'll regret. Also, make it clear to the bride that you'll keep her man in line during his last night of drunken debauchery.
We had two strippers at my brother's party. God, what babes! They handcuffered him, lap danced the poor guy, yada, yada, yada. The best part was the $5 tequila shots. Naked girl lies down, places full shot glass on her tummy, places salt on her left inner thigh, lime juice on her right inner thigh, this is about 2 inches from her you know what, you suck the tequila out of the glass, and then lick the salt and lime juice from either thigh. You should have seen us guys -- like primal animals, faces covered with lime, salt, tequila. Animals...
The guy who's house we had the party at made the mistake of inviting 20 girls over for another party after the strippers left. Those girls were lucky to escape with their lives.
mattja