One of them is the Danish physicist Henrik Svenmark, who hypothesizes that solar activity plays a strong role (beyond the obvious one that the sun warms the Earth). In light of his highly educated guess, it's interesting to note a recent report that sunspots have, for the first time in 100 years, entirely disappeared for an entire month, after tapering off before that.
Perhaps coincidentally, perhaps not, we are currently going through a global cold snap that is predicted, even by the advocates of AGW*, to last for several more years.
In the past 1000 years, three previous such events -- the Dalton, Maunder, and Spörer Minimums, have all led to rapid cooling. On was large enough to be called a "mini ice age". [Daily Tech.]But the relationship is a little less direct than that. According to Svenmark's theory, as I understand it, sunspot activity correlates with cosmic ray output. That it turn influences the average amount of cloud cover over the Earth, which in turn has an effect on the mean temperature. (Also, see this PDF.)
Whether ultimately correct or not, there is real evidence for it (unlike AGW, which is based chiefly on computer models, when it isn't driven purely by desired political outcomes).
For details on the evidence, there are literally thousands of sources. One relatively easy-to-follow one is contained at a blog called The Reference Frame, which references video and scientific papers on the subject.
Science, not politics, will ultimately provide the answer to what causes climate change. But for an easy-to-follow discussion of both, I highly recommend Nigel Lawson's An Appeal to Reason: A Cool Look at Global Warming.
I'll be writing a full review of the book soon, but in the interim I'll tip my hand a bit. It is the only book I know that takes the AGW crowd's views at face value (particularly the IPCC and strictly for the sake of argument), and still demolishes their conclusions and proposals.
[* AGW is an acronym standing for Anthropogenic Global Warming, the hypothesis that humans contribute the major factor (by outputting large amounts of CO2) that have caused average worldwide temperatures to increase, and will do even more so over the next 100 years.]