[cyberax]

Glycol is cheap and safe, but it has lower specific heat capacity and higher viscosity. So that's why water is still being used.

The next step is probably evaporative cooling, with liquid coolant ("freon") pumped to individual racks.

[jabl]

Not sure what Google specifically is using here, but PG25 (25% propylene glycol, 75% water) is somewhat common in data center applications. The glycol takes care of avoiding microbial growth.