It's Ecstasy: This is probably what it is according to most movies where a chick hands a guy a pill in a bar. You take the pill, and you start feeling awesome and if you keep hanging out with this chick you'll probably hook up with her. If she ditches you you'll probably be mugged. Stay hydrated and you'll be all right.

It's a Roofie: Well if you're a hot chick I would recommend never taking a pill from any creepy looking guy, no matter how much they stress that, "It's certainly not a roofie." 9 times out of 10 this guy is lying.
Guys on the other hand, yea you may die from a roofie, but if this chick is hot, I mean, and she's trying to rape you; that would be an awesome story and in my opinion: worth it.

It's Tamiflu: Swine Flu is a big concern lately and who knows? Maybe she's a pharmacist and she walks around raves treating people. Now, this pill is more commonly in a pill shape and has yellow on half of it, while ecstasy more often then not has a picture of something you'd see on a pair of Claire's earrings.

It's Birth Control: Well, for starters, that chick is probably a man; and you either are a woman or look frighteningly so like one. Either way that man/chick is a weirdo, so 9 out of 10 cases of this happening the best thing you can do is to just run far away.

It's an M&M: This is chocolate with a candy shell. Completely harmless, unless you mistake it for Ecstasy and go crazy and then realize that that chick is laughing at you, so you walk over and you're like, "What's up?" and she's like, "I gave you an M&M." and she and her friend start pointing and laughing at you.
That's when you do one of two things, you either say that you knew and that you brought your own ecstasy, or you knock her out and get arrested.

It's a Cheerio: I don't know why someone would be handing out breakfast cereal at a club, but you should notice that you are holding a Cheerio, because 9 times out of 10 a pill is not doughnut shaped, while a Cheerio is.

It's a Thumbtack: If you take one piece of advise from this article; please remember that you should ALWAYS look at the pill before you swallow it, because it may be a thumbtack. 9 times out of 10 swallowing a thumbtack results in instant death.

It's a Lego Figurine's Head: My dogs swallow these all the time and I know from experience that they just pass right through.

It's Your Cell Phone: You must have dropped it and she found it and is giving it back. Don't swallow this! Instead, thank her by flirting with her relentlessly for the rest of the night.

It's One of the Marble Eyes From Your Favorite Childhood Stuffed Animal: First of all, don't immediately freak out and shout out it's embarrassingly noob name, like Mr. Flufficans. Act calm and collected, and ask her what she wants. Follow the clues, enlist your tech-savvy friend, and you should get it back without too much of a hassle or bloodshed.

It's a Skittle: If you suspect foul play but your newly received pill isn't chocolate, than it's most likely a skittle. If you get yellow or orange then throw that shit away but if you get red or green than you're in luck. Enjoy.

IN CONCLUSION: Girls; never swallow anything you're handed at a club. Guys; always swallow anything you're handed at a club.