And it's hard to overstate how huge this is. If you need to write a server that transports gigabytes of data in a stream, and you want to reuse IO pipelines the same way for small data and data-too-large-to-malloc, Quasar's fibers make that possible. Without them, you're stuck in either two-implementations land or callback hell.
It's pretty cool to take an existing Dropwizard app, change a couple of lines and every request becomes a lightweight fiber:
http://docs.paralleluniverse.co/comsat/
http://docs.paralleluniverse.co/quasar/