Hansen is certainly one of the most important climate scientists of the last few decades. Google Scholar credits him with nearly 35,000 citations and an h-index of 78.
The h-index attempts to provide a measure of the productivity and impact of a scholar's published work. Although it is an imperfect measure, in the atmospheric sciences, values above 20 generally indicate a productive senior scientist and above 30 an exceptional senior scientist. 78 is off the scale. Hansen's many important papers can be accessed here.
Publicly, Hansen is perhaps most famous for the testimony he gave to Congress about global warming during the summer of 1998, which after a short introduction began with the following statement.
"I would like to draw three main conclusions. Number one, the earth is warmer in 1988 than at any time in the history of instrumental measurements. Number two, the global warming is now large enough that we can ascribe with a high degree of confidence a cause and effect relationship to the greenhouse effect. And number three, our computer climate simulations indicate that the greenhouse effect is already large enough to effect the probability of extreme events such as summer heat waves."The full text of the testimony can be accessed here and press coverage following the event in this 1988 New York Times article.
An interesting discussion of Hansen's 1988 testimony, subsequent battles with the US government over censorship, and his increased political advocacy is provided at Roger Pielke Jr.'s political blog.