As announced last week and expected for this week, Intel today launched their line of power-efficient quad-core Xeons, with impressive TDPs as low as 50W. In the press release, Intel states that with the power requirement of only 12.5W per core represents a 10-fold reduction in power requirements in just under two years, making it one of the most significant advances in the performance-per-watt arena for a long time:

In addition, these new processors represent a nearly ten-fold improvement in power consumption per core in just 1½ years.*** The company attributes this collective success to the merits of its breakthrough Intel® Core™ microarchitecture and aggressive design execution.
This of course all comes as the industry, from the enterprise level on down to the home consumer, has demanded and pleaded for more power-efficient computing. They'll be introducing the L5320 and L5310 Xeons, which operate at 1.86GHz and 1.6GHz and sport 8MB of on-die cache. For being quad-core server CPUs, they are actually surprisingly cheap - $519 and $455. Intel expects retail availability within a few months.