Intel Atom
The current reigning king of low-cost, low-power processors, Intel's Atom flat-out dominates the netbook market. Its single- and dual-core processors are also some of the most powerful on our list, despite having abilities roughly equal to, in Intel's own terms, a 2003-2004 vintage Celeron. Based on the x86 architecture, the Atom is capable of running full versions of Windows XP, Vista (though not all that well), and 7, as well as modern Linux distros and even Hackintosh. While it requires far less power than a full-power chip, it's still more power-hungry than the ARM-based processors on our list, requiring about 2 watts on average. That's why netbook battery life isn't all that much longer than that of a normal laptop.
You can find the Atom in just about every netbook, including those from HP, Dell, Asus, Acer, Sony, Toshiba, MSI, and, well, everyone else. The 1.6GHz chip is the most popular at the moment, but Intel is definitely going to keep improving and upgrading the Atom line. However, you're unlikely to catch an Atom in a handset; it's low-power, yes, but low-power for a notebook. Battery life on an Atom handset would be pretty atrocious, which is why Intel's sticking to netbooks for now.
Qualcomm Snapdragon
Based on ARM, which is a 32-bit processor architecture that powers just about every mobile phone (and various other peripherals, though never desktop computers) out there, Snapdragon isn't competing directly with the Intel Atom—it's not capable of running full versions of Windows (only Windows Mobile and Windows CE), it's incredibly energy-efficient (requiring less than half a watt), and is designed for always-on use. In other words, this is the evolution of the mobile computing processor. It's got great potential: Qualcomm is trumpeting battery life stretching past 10 hours, smooth 1080p video, support for GPS, 3G, and Bluetooth, and such efficiency that a Linux-based netbook can use Snapdragon without a fan or even a heat sink. Available in single core (1GHz) or dual-core (1.5GHz), it can be used in conjunction with Android, Linux, and various mobile OSes.
Unfortunately, Qualcomm is still holding onto the notion that people want MIDs, and is championing "smartbooks," which are essentially smartphones with netbook bodies, like Asus's announced-then-retracted Eee with Android. Snapdragon's got promise, but we think that promise lies in super-powered handheld devices, not even more underpowered versions of already-underpowered netbooks.
We're frankly not sure when we'll see Snapdragon-based devices sold in the US. We're sure Snapdragon will end up in smartphones at some point, as at least one Toshiba handset has been tentatively announced, but the only concrete demonstrations we've seen have been in MIDs, and Snapdragon themselves spend all their energy touting these ! "smartbo oks." Snapdragon's Windows Mobile compatibility suggests we may see it roll out with Windows Mobile 7, if Tegra hasn't snapped up all the good handsets.
Nvidia Tegra
Nvidia's Tegra processor is very similar to Snapdragon—both are based on ARM architecture, so both are designed for even less intense applications than the Atom. Like Snapdragon, Tegra isn't capable of running desktop versions of Windows, so it's primarily targeted at Android and handheld OSes, especially forthcoming versions of Windows Mobile. What sets Tegra apart from Snapdragon is the Nvidia graphics pedigree: The company claims smooth 1080p video, like Snapdragon, but also hardware-accelerated Flash video and even respectable gaming (though no, you won't be able to run Crysis). They also go even further than Qualcomm in their battery life claim, suggesting an absolutely insane 30 hours of HD video.
While Snapdragon tends to be loosely associated with Android, Tegra is an integral part of Microsoft's plan for next-generation Windows Mobile devices. Instead of focusing on "smartbooks" and MIDs, which we think are part of a dead-end category, Tegra's commitment to pocketable handhelds could spell success. We've seen proof-of-concept demonstrations of Tegra already, but its real commercial debut will come with Windows Mobile 7—and if WM7 doesn't suck, Tegra could take off.
Others
We haven't included certain other processors, especially VIA's Nano, due to intent: The Nano requires lower power than full-scale processors, but at 25 watts, it's not even really in the same league as Atom, let alone Snapdragon or Tegra. The VIA Nano is really targeted at non-portable green technology, and looks like it'll do a good job—it outperformed Atom in Ars Technica's excellent test, and stands up to moderate use with ease. AMD's Puma (Turion X2) is in a similar boat: It's certainly markedly more energy-efficient than AMD's other offerings, but as it's targeted at laptops (not netbooks) with a screen size greater than 12-inches, it's not quite right for our list here.
These low-power processors aren't just, as we so often think, crappier versions of "real" processors. They've got uses far beyond netbooks, especially in the near future as the gap between netbooks and smartphones narrows.
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