I use GNU Parallel. I like it because its interface is simple - input is piping filenames to it, just like xargs, and output is nicely collated to the screen.

I used to use ppss, which does the core task just as well, but the interface is more complex.

I mostly use these tools to optimize large numbers of PNGs before deployment, using optipng, pngout, and/or my own lossypng. These programs take a while to run so using all my cores gets the job done a lot quicker.