In 1937 an amateur American astronomer named Grote Reber completed construction of a 9 meter radio telescope in his back garden. By 1940 he had verified that there were radio signals coming from the heavens and by 1943 he had completed a radio frequency map of the sky. Reber, with his enormous hand-built dish, kick started radio astronomy and eventually sold his invention to the US government.
One of the biggest advances in the understanding of genetic inheritance was made by Gregor Mendel. Mendel was a Augustinian priest. In 1866 his paper on plant hybridization (Mendel had spent years observing and experimenting with pea plants) showed the existence of dominant and recessive genes. Mendel's discoveries went largely unnoticed until rediscovered two professional scienstists.
Amateurs like Reber and Mendel have made enormous contributions to science ever since science was called natural philosophy. So it's dismaying to see a professor from Oxford University write in The Guardian: "The most effective people at finding errors in scientific reasearch are scientists: it was professional glaciologists, after all, who exposed the error in the IPCC 2007 case study of Himalayan glaciers." To exclude the amateur is to deny a large part of the history of science.
Another amateur who made a big impact on science is Albert Einstein. Although Einstein had received a scientific education (after having been refused entry to the prestigious ETH Zurich) he was unable to find a research post and did all his pioneering work while working for the Swiss Patent Office.
I'm no Reber, Mendel or Einstein, but don't rule us amateurs out. In December 2009 the Met Office released thousands of records of temperature readings from around the globe stretching from the present day to 1850. These records form a vital part of the evidence that the globe is warming and the climate changing.
I thought it would be a fun hobby project to use those records to reproduce the worrying charts that show the increase in global temperatures. Since I'm a professional computer programmer I wrote software to process the Met Office data. You can see the result in this YouTube video.
Because I was working with unfamiliar data I put special functions into my program to ensure that I wasn't making any mistakes. To my surprise these functions began reporting that there was something wrong with temperature data in Australia and New Zealand. I whipped out a pocket calculator and checked that my program wasn't mistaken and then reported the problems to the Met Office. They quickly acknowledged that I was right.
Last month the Met Office released an update to CRUTEM3 and HADCRUT3, the critical data sets used to track global warming. The new version contains corrections for all the errors I reported.
Making a distinction between professional and amateur in science is artificial: what matters is the 'what' of science not the 'who'. And amateurs have by their very nature something that professionals don't need to have: passion. Without the comfort of a tenured position, a subsidized bed in an ivory tower, or a well funded laboratory, the only thing keeping amateurs going is a love for their subject.
PS In the comments a professional scientist wrote to object to my final paragraph. I urge you to read his comment since it makes good points. In my defense I didn't mean to say that professional scientists lack passion, just that that's all the amateurs have got. His point is that professionals need to have passion because the funding environment for science is so bad that they're certainly not in it for the money or security!