The gist pointed out that Hash.new, with its default values, is good for implementing Fibonacci sequences. So I decided to check:
fib = Hash.new do |hash,key|
k = key.to_i
hash[key] = case k
when 0 then 0
when 1 then 1
else hash[k-1] + hash[k-2]
This recursive (and memoized!) definition means that you can do fib and get back 2584, plus you get the Fibonacci numbers from F0 (fib) to F17 (fib). Recursively defined Hashes are useful auto-memoized structures.
The Internet seriously rules. Sometimes.