The new DivX v7 is the first video player (that I know of) to support dual-core CPUs. This is a major development as all previous software was unable to coordinate both halves of a now-typical dual-core. So you had to have a really fast CPU to play high-definition videos especially if they were encoded in the new h.264 format. Now, with DivX 7 you can manage it with a much slower dual-core, i.e., with older machines.
Unfortunately, this is only available on Windows (not Linux).
I've done some quick testing and so far the program lives-up to it's claims! I was able to play a 1080p h.264 clip on a dual-core AMD Turion 1.7 GHz, which is about a 3 year-old model. With all previous programs this processor could only play it at half-speed. Now, it can just barely play it at full-speed, and only if the video is full-screen (not windowed) and without anything else displayed on screen such as the mouse pointer.
One problem, however, is that it does not appear to include any deinterlacing. So even though it's fast enough to play 1080i AVCHD clips they look terrible because of the interlacing artifacts.