Windows/Mac: DivX, the well-established maker of video codecs, offers a free web plug-in that decodes Flash video with graphics hardware acceleration. In some cases, especially on netbooks or slower machines, it may do the job better than Adobe's own tools.
DivX's second beta of its web plug-in adds a little black-and-blue play button underneath videos it can help with — usually on YouTube.com, as well as some other free video sites, though not on DRM-wrapped videos. DivX pitches its own video playback as more processor-efficient than Adobe's own beta-level, hardware-assisted 10.2 version. You'll have to be the judge of that — CNET's Scott Rafer suggests the DivX player seemed to help most on an older system, and I tend to see perhaps a smidge of improvement on my two-year-old ThinkPad when watching the CPU performance.
DivX's web plug-in is part of a package that's free for download — you should be able to simply install the web assistance as its own tool. It's available for Windows and Mac systems. Tell us if you see any improvements on your own system using the player in the comments.