Handling our gear's heat has always been an issue for installations large and small. ICT equipment typical took 1x-2x again more energy to remove its heat as it took to power it in the first place (PUE of 2.0+), driving both energy costs and carbon footprints. Early efforts focused on the two obvious tactics: make both the gear and the air conditioning more efficient. We now see totally new approaches to the problem coming online.

'Free cooling', 'air side cooling', or 'air side economization' goes beyond efficient chilling of recirculated interior air to use of outside air to provide low-energy cooling. This has made locations where the air is below 13°C most hours of the year particularly attractive.

Running equipment rooms under 25°C has always been the standard of data center cooling, but new technologies and operating practices are pushing that limit. This allows for both less cooling using conventional techniques and the use of free air at higher exterior temperatures. The American Society of Heating, Refrigerating and Air Conditioning Engineer's Technical Committee 9.9 on Mission Critical Facilities, Technology Spaces and Electronic Equipment (ASHRAE  TC9.9) has raised it's recommended maximum to 27°C, but the industry is already pushing beyond that.  sgi/Rackable, for example, claims its Cloudtrack C2 cabinet can support server room temperatures up to 40°C and Microsoft's new Dublin data center is said to be running at 35°C.

Finally, innovative recycling of waste heat is a growing option, from warming municipal pools to greening greenhouses to heating homes.

The Google/Belgium and Microsoft/Dublin data centers both combine higher operating temperatures and air side cooling to eliminate the massive air conditioning units (chillers) typically associated with large data centers. They are referred to as 'chillerless' data centers. (See more about these and other mega data centers.)

You can see examples of various hot/cold aisle containment techniques in this article's slideshow of German data centers.

Update 2010.02.11:
According to HP, its newest UK data center uses the "legendary cold wind blowing off England's North Sea. This glacier-cooled coastal air, often bone-chillingly icy, is being innovatively harnessed into a new technology tool: lowering temperatures of IT equipment and plant rooms for an anticipated annual energy saving of 40 percent compared to conventional data centres." HP lists these features:
- Eight 2.2m diameter fans in each of the four halls in the data centre used to supply air and another eight used to exhaust air
- A mixing chamber in the facility recirculates air to maintain conditions in the 5m-high pressurised plenum below the computer equipment
- Humidification and cooling coils in the data centre to tune the outside air condition and remove contaminants
Visit HP's Wynyard data center page for more information and a virtual tour.

Update 2010.02.23:
Innovative cooling solutions are not just for mega-data centers. Associated Banc-Corp, a regional bank serving the upper midwestern United States, converted cooling at two data centers from compressors to heat exchangers and expects to save US$115,000 per year in energy costs.

Update 2010.02.26:
Voonami (UT-USA) has a new data center using evaporative cooling that is "along with other engineering innovations, …expected to trim energy costs by 80 percent over a typical giant data center." The technology, particularly suited to dry climates, is planned to be the sole cooling mechanism about eight months of the year.

Update 2010.05.12:
Google typically runs its data centers at ~80 F."

Update 2010.06.03:
Innovative cooling technology alone will not lower energy costs if not properly operated. US EPA data suggests many data centers to not properly operate their economizers.