In my opinion, the "hockey stick chart" is the dominant icon of the Anthropogenic Global Warming (AGW) debate. It was produced by Michael Mann of Penn State in 2001 and relied on by a number of prominent scientists and government agencies, but was challenged and eventually abandoned.
What intrigues me about the hockey stick chart is its reliance on temperatures since 1960 and on other measurements (such as ice cores and tree rings) prior to that. The correlation between tree ring data and measured temperatures is fundamental to this model, which makes the following graph of tree rings and temperatures somewhat sensational:
Insofar as the "hockey stick" represents what people are debating when they talk about "Global Warming," I'd say it's definitely "Mann-made."