This obituary, by Carol Kaesuk Yoon, is a great look at Mayr's accomplishments in evolutionary biology and ornithology. I read his book, What Evolution Is, about a year ago. It's an excellent overview of evolution and natural selection. People more familiar with the literature than I am say it's among the best introductory texts on the subject.
Further, you just gotta admire a man who not only lives to 100 but continues to work and write. His What Makes Biology Unique was published last year by Cambridge University Press.
Mayr also served as an early mentor for the UCLA geographer and physiologist Jared Diamond, author of Guns, Germs, and Steel: The Fates of Human Societies, and Collapse: How Societies Choose to Fail or Succeed.