There are two new chess engines available that have shown to be very strong. Stockfish 1.6.2 JA readily outforms Rybka 2.2n2 (strongest multi-processor version of Rybka available for free) on the CEGT computer rating list. The other engine is RobboLito 0.085g3. This engine is still in preliminary stages of testing, but some say that it is even stronger than the newest version of Rybka (Deep Rybka 3.0).
For more information, I have made the appropriate updates to my "Free Chess Programs" article. Scroll down to the "Playing Software and Engines" section. If you have visited this article recently, you may want to refresh the page.