Zipf's law comes up in so many contexts, from estimating the length of software to the odds of seeing a state's license plate while on family road trips to the size of metropolitan areas.
Back to back of the envelope estimation, when I used to teach CSci 1 Critical Thinking and Computer Science I had the students read Douglas Hofstadter's "On number numbness". I've talked about that chapter and innumeracy before.
The CSci 1 students liked this excerpt:
I once taught a beginning physics class on the thirteenth floor of Hunter College in New York City. From the window we had a magnificent view of the skyscrapers of midtown Manhattan. In one of the opening sessions, I wanted to teach my students about estimates and significant figures, so I asked them to esimate the height of the Empire State Building. In a class of ten students, not one came within a factor of two of the correct answer (1,472 feet with the television antenna, 1,250 without). Most of the estimates were between 300 and 500 feet. One person thought 50 feet was right - a truly amazing underestimate; another thought it was a mile. It turned out that this person had actually calculated the answer, guessing 50 feet per story and 100 stories or so, thus getting about 5,000 feet. Where one person thought that each story was 50 feet high, another thought the whole 102-story building was that high. This startling episode had a deep effect on me..
Three unrelated bonus topics: stand up meetings became popular in Silicon Valley and are regularly rediscovered, and how to use your phone to capture and transcribe meeting notes from a whiteboard, and Gartner's 2009 hype cycle for HCI.