I second Drano.
But if you don't want to use chemicals, maybe try baking soda and vinegar? I've heard this works although I haven't tried it myself. I will tonight though...my shower drain is horribly clogged -.-
Vinegar and Baking soda is a waste of time it won't work
Drano is crystal sodium hydroxide mixed in with bits of aluminum. The Sodium Hydroxide emulsifies the fatty greasy crap in a clog and softens hair and other organic material. The aluminum bits in the Drano react with the sodium hydroxide to form heat which speeds up the emulsfication process. That stuff is corrosive as hell and can eat through aluminum. Easy off oven cleaner has sodium hydroxide in it to eat up greasy oven residue. Sometimes the drano just sits on top of a clog and crystalizes in a rock hard mass and makes the clog worse. A snake with spinning blade can clean a pipe stuck with oatmeal hard crud in the pipe. Be glad its not your sewer line you'll pay almost anything when you flush and see the turds you made come floating to the top of the bowl, you will have a major panic attack..