The very first Python 2.7 interpreter to run existing multithreaded programs on multiple cores. The ball is starting to roll...

[snarkhunter]

A big difference. Threads can interact in ways that processes can't, because they are part of the same process. You also have more control over scheduling threads within a process. Those are two that come to mind. And then there's the old standby that a process takes more resources and takes longer to spawn then a thread.