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]
end
end
This recursive (and memoized!) definition means that you can do fib[18] and get back 2584, plus you get the Fibonacci numbers from F0 (fib[0]) to F17 (fib[17]). Recursively defined Hashes are useful auto-memoized structures.
The Internet seriously rules. Sometimes.