We posted about Nicholas Chapman's Indigo (a free unbiased raytracer) here on BlenderNation back when it was just a newborn version 0.3. They grow up so fast! The latest version 0.4 was released on April 9th and is for Windows only (whose processor must support SSE). There is a Blender export script available for Indigo 0.4 thanks to Nick Chapman and Ewout Fernhout.
And what is an unbiased raytracer you ask? I found a great explanation on Nexlimit's Maxwell site:
Unbiased means that with sufficient render time the rendered solution will always converge to the correct result without the introduction of artifacts. Other rendering engines based upon widely known techniques such as photon mapping, radiosity, light maps, irradiance maps or other interpolation methods always produce biased renders. Consequently, they cannot guarantee convergence to the correct solution no matter how much time is expended on the rendering process.
StompinTom over at the new Blenderartists (elysiun) forums made a test render using Indigo recently that lead to a few interesting points about unbiased renderers that is worth mentioning. Here is what he said:
An unbiased renderer like Indigo leaves no artifacts and is therefore IMO more 'stable' or 'trustworthy' (you know what I mean) meaning I can hit render and go to bed. The noise sucks, yes, but there's a trade off for everything, and, given enough time, the noise clears up. you can get rid of it completely by rendering something twice the size you want it then rescaling it. The ability to almost immediately see what the scene lighting will be like is awesome for tweaking lighting and material settings, quick previewing. Also stopping it whenever I'm satisfied is better than setting a quality and hoping it will be enough IMO. Also, no antialiasing needed... [and]the lighting is incredibly realistic. Since it is unbiased, there aren't as many fakes involved allowing for a more physical light simulation. Finally, probably most importantly, I'm not an animation guy, haven't really had time to get much into that. I like a good solid still shot, and I will wait a week to get it, if that's what it takes. Having said all that, I'm not at all opposed to biased renderers and still use blender internal, yafray, kerkythea, etc.
So if you're looking for unbiased, take a look at the latest Indigo. If you have any renders with it, throw them up at Blenderartist.org!