balance
This is the only thing of any actual use that I wrote in Ruby. Given a space-delimited ledger of credits and debits, it keeps track of current and available funds.
Array#each_n
This mixes in to Array, providing each_n and collect_n methods to iterate over an array n elements at a time.
Hash#setkeys
This near-one-liner mixes in to Hash to allow slice-like assignment in the form:
hash.setkeys(array_of_keys, array_of_values)