With the release of the new Deep Rybka 4 (which can be purchased commercially), the Rybka team also released a multi-processor version for free download. The previous free version was Rybka 2.2n2, and we can now add Rybka 2.3.2a to the list. This engine is slightly stronger according to the CEGT rating list, with a rating of 2980. Originally, the single processor version of 2.3.2a was available via a highly obscure method, but the multi-processor version was not. Now both versions are downloadable from the Official Rybka website. You can find a discussion of the old Rybka 2.2n2 in my Free Chess Programs article.