What OSX apps take advantage of multi-core processors?

I got a Mac Pro mostly for the onboard storage (up to 3TB over four drive bays). I'm certainly not taking advantage of most of my Mac's horsepower. It has two dual-core 2.66GHz processors, but how can I find out what apps actually use the additional processor cores? I've noticed some apps show >100% CPU usage in Activity Monitor. I assume that means they're using multiple cores. I'm particularly interested in speeding up video conversion (ffmpegx et al).