You're not alone - many dudes have wondered what to do with the lime. Do I put it in the drink? Does that make me look girly? Do I set it on the bar? Is that rude? Oh no, that hot chick is staring at me and I don't know what to do with the lime! I better not screw this up!

The short answer: when in doubt, throw it out. Or deposit it discreetly into a cocktail napkin and fold over to conceal the wedge of misery.

The long answer: it depends.

• Beer: fruit at your own risk. Corona and other mexican beers are generally acceptable to "enjoy" with the lime dropped and/or squeezed in. The same goes for wheat beers, but only with lemons or orange. If you get a wheat beer that comes with lime, dump the lime, pound the beer, and find a better bar. Any other American or imported beers should never be consumed with lime in or around the drink.
• Vodka, Gin, Rum, and Tequila based mixed drinks: unless the idea of adding the tartness and flavor of lime into whichever mixers are involved in the cocktail in question sounds gross or appalling, feel free to go for broke and squeeze that shit. After squeezing the lime juice into the drink, it's also acceptable to drop the lime wedge in after it. Or feel free to discard the spent piece of lime as described above...cocktail napkin discreetly, or a three-pointer into that trash can across the bar. Do not under any circumstances leave that lime wedge sitting helpless on the rim of your glass. One exception is in the case of tequila shots -- cope with nasty stuff like Cuervo Gold/Montezuma or other rail varieties any way you can (i.e. yes, do use a lime), but do not use lime and/or salt (aka training wheels) under any circumstances with a call or top-shelf tequila unless you want to ruin the round for everyone.
• Whiskey, Scotch, Cognac, Wine, Absinthe, Irish Cream, any other creamy or milky drink, etc: as a general rule, skip the lime. There are a few exceptions regarding sangria or crazy cocktails. But if you're not at a beach, or in the country of origin for the cocktail you've ordered, you have no business attempting to figure out what the locals do on your own. When in doubt, ditch that citrus. However, if you can trust a local expert to give advice without any guff, feel free to ask him what to do in a self-deprecating way. Chances are, anywhere you may be around the world, Americans are still a laughing stock. Even with Obama in office -- that's how badly Bush fucked us up. And do not ask a woman for advice on this matter -- that's tantamount to stopping to ask directions when a girl is in the car, and signifies testicular detachment.
• Frozen/Blended/Pink/Red drinks, anything with an umbrella or big fruit garnish: See "testicular detachment," above. It's too late for you, so you might as well slit your wrists and squirt the lime into the fresh wounds. The only exception to this law of manhood is if you're being served the drink at an appropriate beach, authentic tiki bar, or while being held hostage -- in any of these situations, it is fine to make the most of the opportunity to drink, and just go all out.
• Limeade: throw the lime wedge at the nearest bartender to get their attention, and ask them to spike that shit for you.

All right playa, now go have some drinks!