Martin Gardner died on May 22 at the age of 95. He wrote the 'Mathematical Games' column in Scientific American for 35 years and was an accomplished critic of pseudo-science.
I first became aware of Gardner by way of his work on C. L. Dodgson (aka Lewis Carroll), especially as evidenced in his Annotated Alice (see also his Universe in a Handkerchief). Of all Gardner's works, though, my favourite one is his Whys of a Philosophical Scrivener (wise indeed). Here's a review from the New Criterion (where Gardner himself published several reviews).
If philosophy begins in wonder, then Whys is a great entryway into philosophy. The index is loaded with names like Wittgenstein, Russell, Reichenbach, Carnap, Rudolf Otto, Tillich, Barth and F. R. Tennant, but Gardner never introduces the philosophy cold (in textbook fashion). Instead, he makes the philosophical questions seem like the most natural and enticing explorations on which to embark. He works up to this effect by way of more literary reflections, sometimes making his approach through passages from children's literature.
For this purpose, Carroll and L. Frank Baum (author of the Wonderful Wizard of Oz) seem to have been Gardner's favourites. I particularly like Gardner's use of an episode from The Marvelous Land of Oz in connection with the concept of the numinous (on p. 342 of Whys). In this episode, Tip is startled when the wooden Saw-Horse comes to life. Gardner notes that the Saw-Horse is even more startled, rolling its eyes and "taking a first wondering view of the world in which he had now so important an existence" (Gardner quoting Baum, p. 342). Gardner then asks whether this (and other) quotations have not stirred in the reader "a similar sense of surprise and wonder." "Have you never," he continues, "felt amazed to find yourself not only living in an Ozzy world but, more incredibly, aware of the fact that you are alive?"
It was in Whys that Gardner announced his 'philosophical theism'. He later characterized himself as "a mystic in the Platonic sense."
I have found two nice blog tributes that focus on Whys, one by Julia Galef and one by William M. Briggs (from whom I've learned that Gardner studied at Chicago with Rudolf Carnap).
One of the best on-line resources concerning Gardner can be found at the Cambridge University Press blog, which includes a 5-part interview from 2008.
Here are several obituaries and tributes: the Guardian, the NY Times, the Telegraph, the Washington Post, the LA Times, the Lewis Carroll Society, Discover, the Skeptical Inquirer, one by James Randi, one by Roger Kimball, a video clip at BoingBoing, and several items at Scientific American (inc. a contribution by Douglas Hofstadter).
Here's an older article from the Wall Street Journal (last April 2) and an interview of Gardner by Alex Bellos.
Update (May 26, 2010): More tributes to Gardner are being tracked here (in English & Spanish).