This page has been around a while, but it's well worth the time it takes to read all of them. Just a few:
http://www.eskimo.com/~billb/weird/skepquot.html
"I am not very skeptical... a good deal of skepticism in a scientific man is advisable to avoid much loss of time, but I have met not a few men who... have often thus been deterred from experiments or observations which would have proven servicable." - Charles Darwin
"Doubt everything or believe everything: these are two equally convenient strategies. With either we dispense with the need for reflection." - Henri Poincare
"It is really quite amazing by what margins competent but conservative scientists and engineers can miss the mark, when they start with the preconceived idea that what they are investigating is impossible. When this happens, the most well-informed men become blinded by their prejudices and are unable to see what lies directly ahead of them." - Arthur C. Clarke, 1963
"With most men, unbelief in one thing springs from blind belief in another." - G. C. Lichtenberg
"It's like religion. Heresy [in science] is thought of as a bad thing, whereas it should be just the opposite." - Dr. Thomas Gold
"The easy confidence with which I know another man's religion is folly teaches me to suspect that my own is also." - Mark Twain
"When a man finds a conclusion agreeable, he accepts it without argument, but when he finds it disagreeable, he will bring against it all the forces of logic and reason." -Thucydides
"In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual." - Galileo Galilei
"If you are only skeptical, then no new ideas make it through to you. You become a crotchety old person convinced that nonsense is ruling the world. (There is, of course, much data to support you.) But every now and then, a new idea turns out to be on the mark, valid and wonderful. If you are too much in the habit of being skeptical about everything, you are going to miss or resent it, and either way you will be standing in the way of understanding and progress. " - Carl Sagan
"As a whole, parapsychologists are nice, honest people, while the critics are cynical, nasty people" - Ray Hyman, skeptical scientist, 1985