http://nodejs.org/ Node is similar in design to and influenced by systems like Ruby's Event Machine or Python's Twisted Matrix. Node takes the event model a bit further—it presents the event loop as a language construct instead of as a library... But what about multiple-processor concurrency? Aren't threads necessary to scale programs to multi-core computers? Processes are necessary to scale to multi-core computers, not memory-sharing threads. The fundamentals of scalable systems are fast networking and non-blocking design—the rest is message passing. In future versions, Node will be able to fork new processes (using the WebWorkers API ) which fits well into the current design.
Nov'2015: Dave Winer thinks the way Node does async is a fundamental weakness that will eventually be routed around by a new framework, as JSON replaced XML. PhilJones nodes some work in FunctionalReactiveProgramming.