I vote for FL Studio. It's a music production program that has built in software synthesizers and other audio processing systems. It can make heavy use of the CPU, both for rendering and real-time playback.
On lower end CPUs, it will quickly start stuttering and you'll start dropping frames. I can watch all 4 cores on my CPU spike as I'm rendering in high quality, too.
This is my most CPU intensive program that I use regularly. The demo is fully functional, though you cannot save projects. This might be a roadblock for creating your own test project, if you don't want to buy it (relatively cheap software, comparatively), but it also does come with many example full-length projects built-in, which might be useful. The trial version has no problem opening existing projects, though. On occasion they may be a "CPU hog" (as is common to say in the comments) though my AMD Phenom II 955 X4 tends to handle most of them fine in realtime playback. Rendering, however, usually takes several minutes at a time. Since it's doing lots of heavy math (for audio processing), it's a good companion to video making software tests.