Web Workers. They’re a W3C standard (well, a working draft at the moment) for exactly this, and require no plugins:
This specification defines an API that allows Web application authors to spawn background workers running scripts in parallel to their main page.
With multicore CPUs becoming prevalent, one way to obtain better performance is to split computationally expensive tasks amongst multiple workers. In [one] example, a computationally expensive task that is to be performed for every number from 1 to 10,000,000 is farmed out to ten subworkers.
setInterval() only schedule things to happen later, they don’t run concurrently with anything else.