With the HTML5 Web Workers standard, browsers now have support for Javascript worker threads, which offer the potential for performing heavy computations in the background without causing CPU usage warnings.

metaworker is a library which wraps around the HTML5 Web Workers standard. It provides an API which is very loosely based on Google's MapReduce algorithm. It partitions input, computes sub-solutions and combines sub-solutions into a solution. It's now posted on github in a very alpha form, and includes two motivating examples on usage. An up-to-date version of Firefox and Firebug are required to use the alpha.