The original mixture, according to The Fine Art of Mixing Drinks, is:

8 parts Cognac or Armagnac
2 parts lemon juice
1 part Cointreau or triple sec

Modern mixtures often have equal parts, I prefer roughly 3:1:1

Mix the ingredients in a shaker half full of ice. Strain and serve in a sugar-rimmed glass. Garnish with a strip of lemon rind

Custom version:
4 parts rye bourbon
2 parts cognac
1 part sweet vermouth
1 part dry vermouth
dash of aromatic bitters
1 maraschino cherry

Instructions

Pour bitters, liquors over ice in mixing glass. Stir and strain into martini glass, garnish with a maraschino cherry.

Dive deep: http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/09/21/WI1ORSF9C.DTL

Variations:

A Dry Manhattan is made with dry vermouth instead of sweet vermouth and served with a twist.
A Brandy Manhattan is made with brandy rather than rye.
A Ruby Manhattan is made with port rather than vermouth.