StacklessIO sounds like a washing machine feature.
With the launch of the latest Empyrean Age 1.1.1 patch, EVE Online has enabled a StacklessIO that is apparently capable of handling over 1,000 people in a local conflict. If that isn't impressive check out these numbers:
Normally Jita reaches a maximum of about 800-900 pilots on any given Sunday. On the Friday following the deployment of StacklessIO, 19 September, there were close to 1,000 concurrent pilots in Jita and on the Saturday, 20 September, the maximum number reached 1,400. This is more than have ever been in Jita at the same time. Under our old network technology Jita could become rather unresponsive at 800-900 pilots but on the Sunday, 21 September, it was quite playable and very responsive with 800 pilots, thanks to StacklessIO.
The system is still being broken in as the 1,400 concurrent Jita pilots did bring the server to a crashing halt. However, this type of scaling is impressive and illustrates CCP's ability to bring resources to bear when they are needed. This advancement was only realized because EVE switched to 64 bit architecture. The dev blog describes in good detail how the system was proposed, designed and installed. To read the ins and outs of blade server specifications and how the magic happened, head over to My EVE (subscription required).