Things you can say to get some ketchup handed to you:
1: Can you pass the ketchup, please? (Note: do not use with english teachers, as they may say, "I can, yes," and then fail to do so until you use a different phrase. Nearly everyone else will just hand you the ketchup, though.)
2: Will you pass the ketchup, please? (Note: use tone and body-language to indicate you are not upset, and make sure your "thanks" is cheerful. This phrasing can be used in an angry way, so it's not as good as...)
3: Could you pass me the ketchup, please?
4: Would you pass me the ketchup, please?
5: Pass [me] the ketchup, please? (Note: this is not rude at all, unless you are saying it in an angry fashion. The "please?" at the end makes it okay.)
5a: Please pass the ketchup. (Note: again, tone of voice is key; so long as you aren't saying it with exasperation, and add a "thanks!" when you get it, you're fine.)
6: Could I get some ketchup over here, please? (Note: again, tone of voice distinguishes between grouchy and "hey, need some ketchup here! Thanks!")
7: My kingdom for the ketchup, woe, woe! (Note: use only with people who appreciate melodrama, and make sure to have the back of your hand to your forehead as you declaim.)
EDITED TO ADD: 8. As Codeswitcher notes in the comments, "Might you pass me the ketchup?" is also acceptable, and in American, probably doesn't need a "please" (though you can certainly add one!). It would tend to be seen as formal/British, and therefore "polite by definition" if the body-language and tone of voice were not hostile.
Don't use "May you pass the ketchup" under any normal circumstances.