Computer-computer battles are ubiquitous nowadays, but I mention this one because Houdini, which is at the moment running away with the aforelinked tournament, was mentioned more than once in the Svidler interview and because it's a freebie. Naturally, there are bells and whistles you get when you buy a Rybka or a Fritz, but for just a plain Jane engine, free is good, and as Houdini might actually be the strongest one out there at the moment, that's even better.
HT: Freddie Jones