Fire 1.2 (untested by me) is a very strong open-source chess engine. I just learned of it a couple weeks ago, and had hesitated to post a link because the Rybka team has claimed that it's based on a reverse compile of Rybka. Since then, however, I've seen no evidence supporting this claim, and know of no pending legal claims in any jurisdiction. (If credible evidence is provided, I will promptly remove the following link.) And I just heard GM Peter Svidler vouch for its strength on Chess FM.
It's free, and it's extremely strong. You can download here.