The simple answer is 100º Celsius, however that depends on where you are and the oxygen content of the water. To explain this better we should look at the molecular structure of H2O. It is a polar covalent bond meaning the oxygen is in the centre and shares one electron with each of the hydrogen's one electron. As there is an unequal amount of electrons this causes a dipole effect giving the hydrogen molecules a small positive charge and the oxygen slightly negative, resulting in a bent shaped molecule. This hydrogen bonding is important in giving water its unique properties and explains why the molecules are attracted to each other (see Figure1)
Figure 1, Hydrogen Bonding
Water is said to be something of an anomaly as compared to other substance it has a very high boiling point and a very low freezing point. This type of hydrogen bonding needs a lot of energy in order turn from a liquid to a gas. As humans we are mostly water and investigation of its properties has long since been undertaken.